How Light Works 22 by nafees

7/25/2019 How Light Works 22 by nafees

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How Light Bulbs Work

Before the invention of the light bulb, illuminating the world after the

sunwent down was a messy, arduous, hazardous task. It took a bunch

of candlesor torchesto fully light up a goodsized room, and oil lamps,

while fairly e!ective, tended to leave a residue of soot on anything in their

general vicinity.

When the science of electricity really got going in the mid "#$$s,inventors everywhere were clamoring to devise a practical, a!ordable

electrical home lighting device. %nglishman &ir 'oseph &wan and (merican

)homas %dison both got it right around the same time *in "#+# and "#+,

respectively-, and within / years, millions of people around the world had

installed electricallighting in their homes. )he easytouse technology

was such an improvement over the old ways that the world never looked

back.

)he amazing thing about this historical turn of events is that the light bulb

itself could hardly be simpler. )he modern light bulb, which hasn0t

changed drastically since %dison0s model, is made up of only a handful of

parts. In this article, we0ll see how these parts come together to produce

bright light for hours on end.

Light Basics

Lightis a form of energy that can be released by an atom. It is made up of

many small particlelike packets that have energy and momentum but no
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mass. )hese particles, called light photons, are the most basic units of

light. *1or more information, see How Light Works.-

(toms release light photons when their electrons become e2cited. If

you0ve read How (toms Work, then you know that electrons are the

negatively charged particles that move around an atom0s nucleus *whichhas a net positive charge-. (n atom0s electrons have di!erent levels of

energy, depending on several factors, including their speed and distance

from the nucleus. %lectrons of di!erent energy levels occupy di!erent

orbital3s. 4enerally speaking, electrons with greater energy move in

orbitals farther away from the nucleus. When an atom gains or loses

energy, the change is e2pressed by the movement of electrons. When

something passes energy on to an atom, an electron may be temporarily

boosted to a higher orbital *farther away from the nucleus-. )he electron

only holds this position for a tiny fraction of a second5 almost immediately,

it is drawn back toward the nucleus, to its original orbital. (s it returns to

its original orbital, the electron releases the e2tra energy in the form of aphoton, in some cases a light photon.

)he wavelength of the emitted light *which determines its color- depends

on how much energy is released, which depends on the particular position

of the electron. 6onse7uently, di!erent sorts of atoms will release

di!erent sorts of light photons. In other words, the color of the light is

determined by what kind of atom is e2cited.

)his is the basic mechanism at work in nearly all light sources. )he main

di!erence between these sources is the process of e2citing the atoms.

In the ne2t section we0ll look at the di!erent parts of a light bulb.
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Light Bulb &tructure

Light bulbs have a very simple structure. (t the base, they have two metal

contacts, which connect to the ends of an electricalcircuit. )he metal

contacts are attached to two sti! wires, which are attached to a thin

metal 8lament. )he 8lament sits in the middle of the bulb, held up bya glass mount. )he wires and the 8lament are housed in a glass bulb,

which is 8lled with an inert gas, such as argon.

When the bulb is hooked up to a power supply, an electric current 9ows

from one contact to the other, through the wires and the 8lament. %lectric

current in a solid conductor is the mass movement of free

electrons *electrons that are not tightly bound to an atom- from a

negatively charged area to a positively charged area.

(s the electrons zip along through the 8lament, they are constantly

bumping into the atoms that make up the 8lament. )he energy of eachimpact vibrates an atom in other words, the current heats the atoms up.

( thinner conductor heats up more easily than a thicker conductor

because it is more resistant to the movement of electrons.

Bound electrons in the vibrating atoms may be boosted temporarily to a

higher energy level. When they fall back to their normal levels, the

electrons release the e2tra energy in the form of photons. :etal atoms

release mostly infrared light photons, which are invisible to the human

eye. But if they are heated to a high enough level around ;,$$$ degrees

1ahrenheit *,$$ degrees 6- in the case of a light bulb they will emit a

good deal of visible light.

)he 8lament in a light bulb is made of a long, incredibly thin length

of tungsten metal. In a typical


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the vibration will break apart the rigid structural bonds between

the atomsso that the material becomes a li7uid. Light bulbs are

manufactured with tungsten 8laments because tungsten has an

abnormally high melting temperature.

But tungstenwill

catch on 8reat such high temperatures, if the conditionsare right. 6ombustion is caused by a reaction between two chemicals,

which is set o! when one of the chemicals has reached

its ignition temperature. =n %arth, combustion is usually a reaction

between o2ygen in the atmosphere and some heated material, but other

combinations of chemicals will combust as well.

)he 8lament in a light bulb is housed in a sealed, o2ygenfree chamber to

prevent combustion. In the 8rst light bulbs, all the air was sucked out of

the bulb to create a near vacuum an area with no matter in it. &ince

there wasn0t any gaseous matter present *or hardly any-, the material

could not combust.

)he problem with this approach was the evaporation of the tungsten

atoms. (t such e2treme temperatures, the occasional tungsten atom

vibrates enough to detach from the atoms around it and 9ies into the air.

In a vacuum bulb, free tungsten atoms shoot out in a straight line and

collect on the inside of the glass. (s more and more atoms evaporate, the

8lament starts to disintegrate, and the glass starts to get darker. )his

reduces the life of the bulb considerably.

In a modern light bulb, inert gases, typically argon, greatly reduce this

loss of tungsten. When a tungsten atom evaporates, chances are it willcollide with an argon atom and bounce right back toward the 8lament,

where it will re>oin the solid structure. &ince inert gases normally don0t

react with other elements, there is no chance of the elements combining

in a combustion reaction.

6heap, e!ective and easytouse, the light bulb has proved a monstrous

success. It is still the most popular method of bringing light indoors and

e2tending the day after sundown. But by all indications, it will eventually

give way to more advanced technologies, because it isn0t very e?cient.

Incandescent light bulbs give o! most of their energy in the form of heatcarrying infrared light photons only about "$ percent of the light

produced is in the visible spectrum. )his wastes a lot of electricity. 6ool

light sources, such as 9uorescent lampsand L%@s, don0t waste a lot of

energy generating heat they give o! mostly visible light. 1or this

reason, they are slowly edging out the old reliable light bulb.

How 1luorescent Lamps Work
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Aou see 9uorescent lighting everywhere these days in o?ces, stores,

warehouses, street corners... Aou0ll even 8nd 9uorescent lamps in peoples0

homes. But even though they0re all around us, these devices are a total

mystery to most people. 'ust what is going on inside those white tubes

In this article, we0ll 8nd out how 9uorescent lamps emit such a bright glowwithout getting scalding hot like an ordinary light bulb. We0ll also 8nd out

why 9uorescent lamps are more e?cient than incandescent lighting, and

see how

Let )here Be Light

)o understand 9uorescent lamps, it helps to know a little about light itself.

Light is a form of energy that can be released by an atom. It is made up of

many small particlelike packets that have energy and momentum but no

mass. )hese particles, called light photons, are the most basic units of

light. *1or more information, see How Light Works.-

(toms release light photons when their electrons become e2cited. If

you0ve read How (toms Work, then you know electrons are the negatively

charged particles that move around an atom0s nucleus *which has a net
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positive charge-. (n atom0s electrons have di!erent levels of energy,

depending on several factors, including their speed and distance from the

nucleus. %lectrons of di!erent energy levels occupy di!erent orbitals.

4enerally speaking, electrons with greater energy move in orbital farther

away from the nucleus.

When atom gains or losses energy, the change is e2pressed by the

movement of electrons. When something passes energy on to an atom

heat, for e2ample an electron may be temporarily boosted to a higher

orbital *farther away from the nucleus-. )he electron only holds this

position for a tiny fraction of a second5 almost immediately, it is drawn

back toward the nucleus, to its original orbital. (s it returns to its original

orbital, the electron releases the e2tra energy in the form of a photon, in

some cases a light photon.

)he wavelengthof the emitted light depends on how much energy is

released, which depends on the particular position of the electron.6onse7uently, di!erent sorts of atoms will release di!erent sorts of light

photons. In other words, the color of the light is determined by what kind

of atom is e2cited.

)his is the basic mechanism at work in nearly all light sources. )he main

di!erence between these sources is the process of e2citing the atoms. In

an incandescent light source, such as an ordinary light bulb or gas lamp,

atoms are e2cited by heat5 in a light stick, atoms are e2cited by a chemical

reaction. 1luorescent lamps have one of the most elaborate systems for

e2citing atoms, as we0ll see in the ne2t section.

@own the )ubes

)he central element in a 9uorescent lamp is a sealed glass tube. )he tube

contains a small bit of mercury and an inert gas, typically argon, kept

under very low pressure. )he tube also contains a phosphor powder,
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coated along the inside of the glass. )he tube has two electrodes, one at

each end, which are wired to an electrical circuit. )he electrical circuit,

which we0ll e2amine later, is hooked up to an alternating current *(6-

supply.

When you turn the lamp on, the current 9ows through the electrical circuitto the electrodes. )here is a considerable voltage across the electrodes,

so electrons will migrate through the gas from one end of the tube to the

other. )his energy changes some of the mercury in the tube from a li7uid

to a gas. (s electrons and charged atoms move through the tube, some of

them will collide with the gaseous mercury atoms. )hese collisions e2cite

the atoms, bumping electrons up to higher energy levels. When the

electrons return to their original energy level, they release light photons.

(s we saw in the last section, the wavelength of a photon is determined

by the particular electron arrangement in the atom. )he electrons in

mercury atoms are arranged in such a way that they mostly release lightphotons in the ultraviolet wavelength range. =ur eyes don0t register

ultraviolet photons, so this sort of light needs to be converted into visible

light to illuminate the lamp.

)his is where the tube0s phosphor powder coating comes in. Chosphors are

substances that give o! light when they are e2posed to light. When a

photon hits a phosphor atom, one of the phosphor0s electrons >umps to a

higher energy level and the atom heats up. When the electron falls back to

its normal level, it releases energy in the form of another photon. )his

photon has less energy than the original photon, because some energy

was lost as heat. In a 9uorescent lamp, the emitted light is in the visiblespectrum the phosphor gives o! white light we can see. :anufacturers

can vary the color of the light by using di!erent combinations of

phosphors.

6onventional incandescent light bulbs also emit a good bit of ultraviolet

light, but they do not convert any of it to visible light. 6onse7uently, a lot

of the energy used to power an incandescent lamp is wasted. (

9uorescent lamp puts this invisible light to work, and so is more e?cient.

Incandescent lamps also lose more energy through heat emission than do

9uorescent lamps. =verall, a typical 9uorescent lamp is four to si2 times

more e?cient than an incandescent lamp. Ceople generally use

incandescent lights in the home, however, since they emit a DwarmerD

light a light with more red and less blue.

(s we0ve seen, the entire 9uorescent lamp system depends on an

electrical current 9owing through the gas in the glass tube. In the ne2t


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section, we0ll see what a 9uorescent lamp needs to do to establish this

current.

6ooking with 4as

In the last section, we saw that mercury atoms in a 9uorescent lamps

glass tube are e2cited by electrons 9owing in an electrical current. )his

electrical current is something like the current in an ordinary wire, but it

passes through gas instead of through a solid. 4as conductors di!er from

solid conductors in a number of ways.

In a solid conductor, electrical charge is carried by free electrons >umping

from atom to atom, from a negativelycharged area to a positively

charged area. (s we0ve seen, electrons always have a negative charge,

which means they are always drawn toward positive charges. In a gas,

electrical charge is carried by free electrons moving independently of

atoms. 6urrent is also carried by ions, atoms that have an electricalcharge because they have lost or gained an electron. Like electrons, ions

are drawn to oppositely charged areas.

)o send a current through gas in a tube, then, a 9uorescent light needs to

have two thingsE

". 1ree electrons and ions

. ( di!erence in charge between the two ends of the tube *a voltage-

4enerally, there are few ions and free electrons in a gas, because all of

the atoms naturally maintain a neutral charge. 6onse7uently, it is di?cultto conduct an electrical current through most gases. When you turn on a

9uorescent lamp, the 8rst thing it needs to do is introduce many new free

electrons from both electrodes.
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&tart it Fp

)he classic 9uorescent lamp design, which has fallen mostly by the

wayside, used a special starter switch mechanism to light up the tube. Aou

can see how this system works in the diagram below.

When the lamp 8rst turns on, the path of least resistance is through the

bypass circuit, and across the starter switch. In this circuit, the current

passes through the electrodes on both ends of the tube. )hese electrodes

are simple 8laments, like you would 8nd in an incandescent light bulb.

When the current runs through the bypass circuit, electricity heats up the

8laments. )his boils o! electrons from the metal surface, sending them

into the gas tube, ionizing the gas.

(t the same time, the electrical current sets o! an interesting se7uence of

events in the starter switch. )he conventional starter switch is a small

discharge bulb, containing neon or some other gas. )he bulb has twoelectrodes positioned right ne2t to each other. When electricity is initially

passed through the bypass circuit, an electrical arc*essentially, a 9ow of

charged particles- >umps between these electrodes to make a connection.

)his arc lights the bulb in the same way a larger arc lights a 9uorescent

bulb.

=ne of the electrodes is a bimetallic strip that bends when it is heated.

)he small amount of heat from the lit bulb bends the bimetallic strip so it

makes contact with the other electrode. With the two electrodes touchingeach other, the current doesn0t need to >ump as an arc anymore.

6onse7uently, there are no charged particles 9owing through the gas, and

the light goes out. Without the heat from the light, the bimetallic strip

cools, bending away from the other electrode. )his opens the circuit.
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Inside the casing of a conventional 9uorescent starter there is a small gas

discharge lamp.

By the time this happens, the 8laments have already ionized the gas in

the 9uorescent tube, creating an electrically conductive medium. )he tube

>ust needs a voltage kick across the electrodes to establish an electrical

arc. )his kick is provided by the lamp3s ballast, a special sort oftransformer wired into the circuit.

When the current 9ows through the bypass circuit, it establishes a

magnetic 8eld in part of the ballast. )his magnetic 8eld is maintained by

the 9owing current. When the starter switch is opened, the current is

brie9y cut o! from the ballast. )he magnetic 8eld collapses, which creates

a sudden >ump in current the ballast releases its stored energy.

Gapid start and starter switch 9uorescent bulbs have two pins that slide

against two contact points in an electrical circuit.

Light Gight (way

)oday, the most popular 9uorescent lamp design is the rapid start lamp.

)his design works on the same basic principle as the traditional starter

lamp, but it doesn0t have a starter switch. Instead, the lamp0s ballast

constantly channels current through both electrodes. )his current 9ow is

con8gured so that there is a charge di!erence between the two

electrodes, establishing a voltage across the tube.


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When the 9uorescent light is turned on, both electrode 8laments heat up

very 7uickly, boiling o! electrons, which ionize the gas in the tube. =nce

the gas is ionized, the voltage di!erence between the electrodes

establishes an electrical arc. )he 9owing charged particles *red- e2cite the

mercury atoms *silver-, triggering the illumination process.

(n alternative method, used in instantstart 9uorescent lamps, is to apply

a very high initial voltage to the electrodes. )his high voltage creates

a corona discharge. %ssentially, an e2cess of electrons on the electrode

surface forces some electrons into the gas. )hese free electrons ionize the

gas, and almost instantly the voltage di!erence between the electrodes

establishes an electrical arc.

o matter how the starting mechanism is con8gured, the end result is the

sameE a 9ow of electrical current through an ionized gas. )his sort of gas

discharge has a peculiar and problematic 7ualityE If the current isn0t

carefully controlled, it will continually increase, and possibly e2plode thelight 82ture. In the ne2t section, we0ll 8nd out why this is and see how a

9uorescent lamp keeps things running smoothly.

)he ballast, starter switch and 9uorescent bulb are all wired together in a

simple circuit.

)his surge in current helps build the initial voltage needed to establish the

electrical arc through the gas. Instead of 9owing through the bypass

circuit and >umping across the gap in the starter switch, the electrical

current 9ows through the tube. )he free electrons collide with the atoms,

knocking loose other electrons, which create ions. )he result is plasma, a

gas composed largely of ions and free electrons, all moving freely. )his

creates a path for an electrical current.

)he impact of 9ying electrons keeps the two 8laments warm, so they

continue to emit new electrons into the plasma. (s long as there is (6

current, and the 8laments aren0t worn out, current will continue to 9ow

through the tube.
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)he problem with this sort of lamp is it takes a few seconds for it to light

up. )hese days, most 9uorescent lamps are designed to light up almost

instantly. In the ne2t section, we0ll see how these modern designs work.

Ballast Balance

We saw in the last section that gases don0t conduct electricity in the same

way as solids. =ne ma>or di!erence between solids and gases is

their electrical resistance *the opposition to 9owing electricity-. In a solid

metal conductor such as a wire, resistance is a constant at any given

temperature, controlled by the size of the conductor and the nature of the

material.

In a gas discharge, such as a 9uorescent lamp, current causes resistance

to decrease. )his is because as more electrons and ions 9ow through a

particular area, they bump into more atoms, which frees up electrons,

creating more charged particles. In this way, current will climb on its ownin a gas discharge, as long as there is ade7uate voltage *and household

(6 current has a lot of voltage-. If the current in a 9uorescent light isn0t

controlled, it can blow out the various electrical components.

1luorescent lamps ballast works to control this. )he simplest sort of

ballast, generally referred to as magnetic ballast, works something like

an inductor. ( basic inductor consists of a coil of wire in a circuit, which

may be wound around a piece of metal. If you0ve read How %lectromagnets

Work, you know that when you send electrical current through a wire, it

generates a magnetic 8eld. Cositioning the wire in concentric loops

ampli8es this 8eld.

)his sort of 8eld a!ects not only ob>ects around the loop, but also the loop

itself. Increasing the current in the loop increases the magnetic 8eld,

which applies a voltage opposite the 9ow of current in the wire. In short, a

coiled length of wire in a circuit *an inductor- opposes change in the

current 9owing through it. )he transformer elements in a magnetic ballast

use this principle to regulate the current in a 9uorescent lamp.

Ballast can only slow down changes in current it can0t stop them. But

the alternating current powering a 9uorescent light is

constantly reversing itself, so the ballast only has to inhibit increasingcurrent in a particular direction for a short amount of time. 6heck out this

sitefor more information on this process.

:agnetic ballasts modulate electrical current at a relatively low cycle rate,

which can cause a noticeable 9icker. :agnetic ballasts may also vibrate at

a low fre7uency. )his is the source of the audible humming sound people

associate with 9uorescent lamps.
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:odern ballast designs use advanced electronics to more precisely

regulate the current 9owing through the electrical circuit. &ince they use a

higher cycle rate, you don0t generally notice a 9icker or humming noise

coming from electronic ballast. @i!erent lamps re7uire specialized ballasts

designed to maintain the speci8c voltage and current levels needed for

varying tube designs.

1luorescent lamps come in all shapes and sizes, but they all work on the

same basic principleE (n electric current stimulates mercury atoms, which

causes them to release ultraviolet photons. )hese photons in turn

stimulate a phosphor, which emits visible light photons. (t the most basic

level, that0s all there is to it

How Gelays Work

( relay is a simple electromechanical switch made up of an

electromagnet and a set of contacts. Gelays are found hidden in all sortsof devices. In fact, some of the 8rst computers ever built used relays to

implement Boolean gates.

In this article, we will look at how relays work and a few of their

applications

Gelay 6onstruction

Gelays are amazingly simple devices. )here are four parts in every relayE

%lectromagnet

(rmature that can be attracted by the electromagnet

&pring

&et of electrical contacts

)he following 8gure shows these four parts in actionE

In this 8gure, you can see that a relay consists of two separate and

completely independent circuits. )he 8rst is at the bottom and drives

the electromagnet. In this circuit, a switch is controlling power to the

electromagnet. When the switch is on, the electromagnet is on, and itattracts the armature *blue-. )he armature is acting as a switch in the

second circuit. When the electromagnet is energized, the armature

completes the second circuit and the light is on. When the electromagnet

is not energized, the spring pulls the armature away and the circuit is not

complete. In that case, the light is dark.
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When you purchase relays, you generally have control over several

variablesE

)he voltage and current that is needed to activate the armature

)he ma2imum voltage and current that can run through the

armature and the armature contacts

)he number of armatures *generally one or two-

)he number of contacts for the armature *generally one or two the

relay shown here has two, one of which is unused-

Whether the contact *if only one contact is provided-

is normally open *=- or normally closed *6-

Gelay (pplications

In general, the point of a relay is to use a small amount of power in

theelectromagnet coming, say, from a small dashboard switch or a low

power electronic circuit to move an armature that is able to switch a

much larger amount of power. 1or e2ample, you might want the

electromagnet to energize using / volts and /$ milliamps */$ mill watts-,

while the armature can support "$J (6 at amps *;$ watts-.

Gelays are 7uite common in home appliances where there is an electronic

control turning on something like a motor or a light. )hey are also

common in cars, where the "J supply voltage means that >ust about

everything needs a large amount of current. In later model cars,manufacturers have started combining relay panels into the fuse bo2 to

make maintenance easier. 1or e2ample, the si2 gray bo2es in this photo of

a 1ord Windstar fuse bo2 are all relaysE

In places where a large amount of power needs to be switched, relays are

often cascaded. In this case, a small relay switches the power needed to

drive a much larger relay, and that second relay switches the power to

drive the load.

How Light Works
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Light as Gays

Imagining light as a ray makes it easy to describe, with great accracy, three well!

known "henomena# re$ection, re%raction and scattering& 'et(s take a second to

discss each one&

In re9ection, a light ray strikes a smooth sr%ace, sch as a mirror, and bonceso)& * re$ected ray always comes o) the sr%ace o% a material at an angle e+al to

the angle at which the incoming ray hit the sr%ace& In "hysics, yo(ll hear this

called the law of re9ection& o(-e "robably heard this law stated as .the angle o%

incidence e+als the angle o% re$ection&.

/% corse, we li-e in an im"er%ect world and not all sr%aces are smooth& hen light

strikes a rogh sr%ace, incoming light rays re$ect at all sorts o% angles becase the

sr%ace is ne-en& his scattering occrs in many o% the obects we enconter

e-ery day& he sr%ace o% "a"er is a good eam"le& o can see st how rogh it is

i% yo "eer at it nder a microsco"e& hen light hits "a"er, the wa-es are re$ected

in all directions& his is what makes "a"er so incredibly se%l !! yo can read the

words on a "rinted "age regardless o% the angle at which yor eyes -iew the

sr%ace&

Gefractionoccrs when a ray o% light "asses %rom one trans"arent medim air,

let(s say to a second trans"arent medim water& hen this ha""ens, light

changes s"eed and the light ray bends, either toward or away %rom what we call

the normal line, an imaginary straight line that rns "er"endiclar to the sr%ace
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o% the obect& he amont o% bending, or angle of refraction, o% the light wa-e

de"ends on how mch the material slows down the light& iamondswoldn(t be so

glittery i% they didn(t slow down incoming light mch more than, say, water does&

iamonds ha-e a higher inde o% re%raction than water, which is to say that those

s"arkly, costly light tra"s slow down light to a greater degree&

'enses, like those in a telesco"eor in a "air o% glasses, take ad-antage o% re%raction&

* lens is a "iece o% glass or other trans"arent sbstance with cr-ed sides %or

concentrating or dis"ersing light rays& 'enses ser-e to re%ract light at each

bondary& *s a ray o% light enters the trans"arent material, it is re%racted& *s the

same ray eits, it(s re%racted again& he net e)ect o% the re%raction at these two

bondaries is that the light ray has changed directions& e take ad-antage o% this

e)ect to correct a "erson(s -ision or enhance it by making distant obects a""ear

closer or small obects a""ear bigger&

n%ortnately, a ray theory can(t e"lain all o% the beha-iors ehibited by light& e(ll

need a %ew other e"lanations, like the one we(ll co-er net&

Light as Waves

nlike water wa-es, light wa-es %ollow more com"licated "aths, and they don(t

need a medim to tra-el throgh&

hen the 19th centry dawned, no real e-idence had accmlated to "ro-e the

wa-e theory o% light& hat changed in 1801 when homas ong, an nglish

"hysician and "hysicist, designed and ran one o% the most %amos e"eriments in

the history o% science& It(s known today as the doubleslit e2perimentand

re+ires sim"le e+i"ment !! a light sorce, a thin card with two holes ct side by

side and a screen&

o rn the e"eriment, ong allowed a beam o% light to "ass throgh a "inhole and

strike the card& I% light contained "articles or sim"le straight!line rays, he reasoned,

light not blocked by the o"a+e card wold "ass throgh the slits and tra-el in a

straight line to the screen, where it wold %orm two bright s"ots& his isn(t what

ong obser-ed& Instead, he saw a bar code "attern o% alternating light and dark

bands on the screen& o e"lain this ne"ected "attern, he imagined light tra-eling

throgh s"ace like a water wa-e, with crests and troghs& hinking this way, he

conclded that light wa-es tra-eled throgh each o% the slits, creating two se"arate

wa-e %ronts& *s these wa-e %ronts arri-ed at the screen, they inter%ered with eachother& right bands %ormed where two wa-e crests o-erla""ed and added together&

ark bands %ormed where crests and troghs lined " and canceled each other ot

com"letely&

ong(s work s"arked a new way o% thinking abot light& :cientists began re%erring

to light wa-es and resha"ed their descri"tions o% re$ection and re%raction

accordingly, noting that light wa-es still obey the laws o% re$ection and re%raction&
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Incidentally, the bending o% a light wa-e acconts %or some o% the -isal "henomena

we o%ten enconter, sch as mirages& * mirageis an o"tical illsion cased when

light wa-es mo-ing %rom the sky toward the grond are bent by the heated air&

In the 1860s, :cottish "hysicist ;ames elds& he >elds -ibrate at right angles to the direction o% mo-ement

o% the wa-e, and at right angles to each other& ecase light has both electric and

magnetic >elds, it(s also re%erred to as electromagnetic radiation&

lectromagnetic radiation doesn(t need a medim to tra-el throgh, and, when it(s

tra-eling in a -acm, mo-es at 186,000 miles "er second 300,000 kilometers "er

second& :cientists re%er to this as the speed of light, one o% the most im"ortant

nmbers in "hysics&

Light waves come in a continuous variety of sizes, fre7uencies andenergies, a continuum known as the electromagnetic spectrum.

Light 1re7uencies

/nce =awell introdced the conce"t o% electromagnetic wa-es, e-erything clicked

into "lace& :cientists now cold de-elo" a com"lete working model o% light sing

terms and conce"ts, sch as wa-elength and %re+ency, based on the strctre and

%nction o% wa-es& *ccording to that model, light wa-es come in many si?es& he

si?e o% a wa-e is measred as its wavelength, which is the distance between any

two corres"onding "oints on sccessi-e wa-es, sally "eak to "eak or trogh to

trogh& he wa-elengths o% the light we can see range %rom 400 to 700 nanometersor billionths o% a meter& t the %ll range o% wa-elengths inclded in the de>nition

o% electromagnetic radiationetends %rom 0&1 nanometers, as in gamma rays, to

centimeters and meters, as in radiowa-es&

'ight wa-es also come in many %re+encies& he fre7uencyis the nmber o% wa-es

that "ass a "oint in s"ace dring any time inter-al, sally one second& e measre

it in nits o% cycles wa-es "er second, or hertz& he %re+ency o% -isible light is
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re%erred to as color, and ranges %rom 430 trillion hert?, seen as red, to 750 trillion

hert?, seen as -iolet& *gain, the %ll range o% %re+encies etends beyond the -isible

"ortion, %rom less than 3 billion hert?, as in radio wa-es, to greater than 3 billion

billion hert? 3 1019, as in gamma rays&

he amont o% energy in a light wa-e is "ro"ortionally related to its %re+ency# @igh%re+ency light has high energyA low %re+ency light has low energy& :o, gamma

rays ha-e the most energy "art o% what makes them so dangeros to hmans, and

radio wa-es ha-e the least& /% -isible light, -iolet has the most energy and red the

least& he whole range o% %re+encies and energies, shown in the accom"anying

>gre, is known as the electromagnetic spectrum& Bote that the >gre isn(t

drawn to scale and that -isible light occ"ies only one!thosandth o% a "ercent o%

the s"ectrm&

his might be the end o% the discssion, ece"t that *lbert insteincoldn(t let

s"eeding light wa-es lie& @is work in the early 20th centry resrrected the old idea

that light, st maybe, was a "article a%ter all&

&olar panels take advantage of the photoelectric e!ect to power our

homes and businesses.

Light as Carticles

=awell(s theoretical treatment o% electromagnetic radiation, inclding its

descri"tion o% light wa-es, was so elegant and "redicti-e that many "hysicists in the

1890s thoght that there was nothing more to say abot light and how it worked&

hen, on ec& 14, 1900, =a Clanck came along and introdced a stnningly sim"le,

yet strangely nsettling, conce"t# that light mst carry energy in discrete +antities&

hose +antities, he "ro"osed, mst be nits o% the basic energy increment, hf,where his a ni-ersal constant now known as Clanck0s constantand fis the

%re+ency o% the radiation&

*lbert instein ad-anced Clanck(s theory in 1905 when he stdied

the photoelectric e!ect& Dirst, he began by shining ltra-iolet lighton the sr%ace

o% a metal& hen he did this, he was able to detect electrons being emitted %rom the

sr%ace& his was instein(s e"lanation# I% the energy in light comes in bndles,
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then one can think o% light as containing tiny lm"s, or photons& hen these

"hotons strike a metal sr%ace, they act like billiard balls, trans%erring their energy

to electrons, which become dislodged %rom their ."arent. atoms& /nce %reed, the

electrons mo-e along the metal or get eected %rom the sr%ace&

he "article theory o% light had retrned !! with a -engeance& Bet, Biels ohra""lied Clanck(s ideas to re>ne the model o% an atom& arlier scientists had

demonstrated that atoms consist o% "ositi-ely charged nclei srronded by

electrons orbiting like "lanets, bt they coldn(t e"lain why electrons didn(t sim"ly

s"iral into the ncles& In 1913, ohr "ro"osed that electrons eist in discrete orbits

based on their energy& hen an electron m"s %rom one orbit to a lower orbit, it

gi-es o) energy in the %orm o% a "hoton&

he +antm theory o% light !! the idea that light eists as tiny "ackets, or "articles,

called "hotons !! slowly began to emerge& /r nderstanding o% the "hysical world

wold no longer be the same&

WaveCarticle @uality

*t >rst, "hysicists were relctant to acce"t the dal natre o% light& *%ter all, many

o% s hmans like to ha-e one right answer& t instein"a-ed the way in 1905 by

embracing waveparticle duality& e(-e already discssed the "hotoelectric

e)ect, which led instein to describe light as a "hoton& 'ater that year, howe-er, he

added a twist to the story in a "a"er introdcing s"ecial relati-ity& In this "a"er,

instein treated light as a continos >eld o% wa-es !! an a""arent contradiction to

his descri"tion o% light as a stream o% "articles& et that was "art o% his genis& @e

willingly acce"ted the strange natre o% light and chose whiche-er attribte best

addressed the "roblem he was trying to sol-e&

oday, "hysicists acce"t the dal natre o% light& In this modern -iew, they de>ne

light as a collection o% one or more "hotons "ro"agating throgh s"ace as

electromagnetic wa-es& his de>nition, which combines light(s wa-e and "article

natre, makes it "ossible to rethink homas ong(s doble!slit e"eriment in this

way# 'ight tra-els away %rom a sorce as an electromagnetic wa-e& hen it

enconters the slits, it "asses throgh and di-ides into two wa-e %ronts& hese wa-e

%ronts o-erla" and a""roach the screen& *t the moment o% im"act, howe-er, the

entire wa-e >eld disa""ears and a "hoton a""ears& Eantm "hysicists o%ten

describe this by saying the s"read!ot wa-e .colla"ses. into a small "oint&

:imilarly, "hotons make it "ossible %or s to see the world arond s& In total

darkness, or eyes are actally able to sense single "hotons, bt generally what we

see in or daily li-es comes to s in the %orm o% ?illions o% "hotons "rodced by light

sorces and re$ected o) obects& I% yo look arond yo right now, there is "robably

a light sorce in the room "rodcing "hotons, and obects in the room that re$ect

those "hotons& or eyes absorb some o% the "hotons $owing throgh the room, and

that(s how yo see&
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t wait& hat makes a light sorce "rodce "hotonsF e(ll get to that& Bet&

Croducing a Choton

here are many di)erent ways to "rodce "hotons, bt all o% them se the same

mechanism inside an atomto do it& his mechanism in-ol-es the energi?ing o%

electrons orbiting each atom(s ncles& @ow Bclear Gadiation orksdescribes

"rotons, netrons and electrons in some detail& Dor eam"le, hydrogen atoms ha-e

one electron orbiting the ncles& @elim atoms ha-e two electrons orbiting the

ncles& *lminm atoms ha-e 13 electrons circling the ncles& ach atom has a

"re%erred nmber o% electrons ?i""ing arond its ncles&

lectrons circle the ncles in >ed orbits !! a sim"li>ed way to think abot it is to

imagine how satellitesorbit the arth& here(s a hge amont o% theory arondelectron orbitals, bt to nderstand light there is st one key %act to nderstand# *n

electron has a natral orbit that it occ"ies, bt i% yo energi?e an atom, yo can

mo-e its electrons to higher orbitals& * "hoton is "rodced whene-er an electron in

a higher!than!normal orbit %alls back to its normal orbit& ring the %all %rom high

energy to normal energy, the electron emits a "hoton !! a "acket o% energy !! with

-ery s"eci>c characteristics& he "hoton has a %re+ency, or color, that eactly

matches the distance the electron %alls&

o can see this "henomenon +ite clearly in gas!discharge lam"s& Dlorescent

lam"s, neon signs and sodim!-a"or lam"s are common eam"les o% this kind o%

electric lighting, which "asses an electric crrent throgh a gas to make the gas

emit light& he colors o% gas!discharge lam"s -ary widely de"ending on the identity

o% the gas and the constrction o% the lam"&

Dor eam"le, along highways and in "arking lots, yo o%ten see sodim -a"or lights&

o can tell a sodim -a"or light becase it(s really yellow when yo look at it& *

sodim -a"or light energi?es sodim atoms to generate "hotons& * sodim atom
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has 11 electrons, and becase o% the way they(re stacked in orbitals one o% those

electrons is most likely to acce"t and emit energy& he energy "ackets that this

electron is most likely to emit %all right arond a wa-elength o% 590 nanometers&

his wa-elength corres"onds to yellow light& I% yo rn sodim light throgh a

"rism, yo don(t see a rainbow !! yo see a "air o% yellow lines&

BioluminescenceE How =rganisms Light )hings Fp

*nother way to make "hotons, known as chemiluminescence, in-ol-es chemical

reactions& hen these reactions occr in li-ing organisms sch as bacteria, >re$ies,

s+id and dee"!sea >shes, the "rocess is known as bioluminescence& *t least two

chemicals are re+ired to make light& shes, also

known as looseaws& hese animals can both "rodce red light and detect it when

other organisms can(t&

ant to know more abot how and why li-ing things make lightF


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IncandescenceE 6reating Light With Heat

Crobably the most common way to energi?e atomsis with heat, and this is the basis

o% incandescence& I% yo heat " a horseshoe with a blowtorch, it will e-entally

get red!hot, and i% yo indlge yor inner "yromaniacand heat it e-en more, it gets

white hot& Ged is the lowest!energy -isible light, so in a red!hot obect the atoms are

st getting enogh energy to begin emitting light that we can see& /nce yo a""ly

enogh heat to case white light, yo are energi?ing so many di)erent electrons in

so many di)erent ways that all o% the colors are being generated !! they all mi

together to look white&

@eat is the most common way we see light being generated !! a normal 75!watt

incandescent blb is generating light by sing electricityto create heat& lectricity

rns throgh a tngsten >lament hosed inside a glass s"here& ecase the

>lament is so thin, it o)ers a good bit o% resistance to the electricity, and this

resistance trns electrical energy into heat& he heat is enogh to make the

>lament glow white!hot& n%ortnately, this isn(t -ery eHcient& =ost o% the energy

that goes into an incandescent blb is lost as heat& In %act, a ty"ical light blb

"rodces "erha"s 15 lmens "er watt o% in"t "ower com"ared to a $orescent

blb, which "rodces between 50 and 100 lmens "er watt&


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(n illustration of a laser

Lasers

*n interesting a""lication o% the +antm natre o% light is the laser& o can get the

whole story on lasers in @ow 'asers ork, bt we(re going to co-er some o% the keyconce"ts here& Laseris an acronym %or .light am"li>cation by stimlated emission

o% radiation,. which is a tonge!tying way to describe light in which the "hotons are

all at the same wa-elength and ha-e their crests and troghs in "hase& Gesearch

"hysicist heodore @& =aiman de-elo"ed the world(s >rst working laser, the rby

laser, in 1960& he rby laser contained a rby crystal, a +art? $ash tbe,

re$ecting mirrors and a "ower s""ly&

'et(s re-iew how =aiman sed these com"onents to create laser light, starting with

the characteristics o% rby& Gby is an alminm oide crystal in which some o% the

alminm atoms ha-e been re"laced with chromim atoms&


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White light is a mi2ture of colors.

:aking 6olors

isible light is light that the hman eye can "ercei-e& hen yo look at the sn(s

-isible light, it a""ears to be colorless, which we call white& *lthogh we can see

this light, white isn(t considered "art o% the -isible s"ectrm& hat(s becase whitelight isn(t the light o% a single color bt instead many colors&

hen snlight "asses throgh a glass o% water to land on a wall, we see a rainbow

on the wall& his woldn(t ha""en nless white light were a mitre o% all o% the

colors o% the -isible s"ectrm& Isaac Bewtonwas the >rst "erson to demonstrate

this& Bewton "assed snlight throgh a glass "rism to se"arate the colors into

a rainbows"ectrm& @e then "assed snlight throgh a second glass "rism and

combined the two rainbows& he combination "rodced white light& @is sim"le

e"eriment "ro-ed conclsi-ely that white light is a mitre o% colors&

o can do a similar e"eriment with three $ashlights and three di)erent colors o%cello"hane !! red, green and ble commonly re%erred to as GJ&


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Cigments are created by modifying which colors are absorbed.

Cigments and (bsorption

*nother way to make colors is to absorb some o% the %re+encies o% light, and ths

remo-e them %rom the white light combination& he absorbed colors are the ones

yo don(t see !! yo see only the colors that come boncing back to yor eye& hisis known as subtractive color, and it(s what ha""ens with "aints and dyes& he

"aint or dye molecles absorb s"eci>c %re+encies and bonce back, or re$ect,

other %re+encies to yor eye& he re$ected %re+ency or %re+encies are what

yo see as the color o% the obect& Dor eam"le, the lea-es o% green "lantscontain a

"igment called chloro"hyll, which absorbs the ble and red colors o% the s"ectrm

and re$ects the green&

o can e"lain absor"tion in terms o% atomic strctre& he %re+ency o% the

incoming light wa-e is at or near the -ibration %re+ency o% the electrons in the

material& he electrons take in the energy o% the light wa-e and start to -ibrate&

hat ha""ens net de"ends "on how tightly the atomshold on to their electrons&

*bsor"tion occrs when the electrons are held tightly, and they "ass the -ibrations

along to the nclei o% the atoms& his makes the atoms s"eed ", collide with other

atoms in the material, and then gi-e " as heat the energy they ac+ired %rom the

-ibrations&

he absor"tion o% light makes an obect dark or o"a+e to the %re+ency o% the

incoming wa-e& ood is o"a+e to -isible light& :ome materials are o"a+e to some

%re+encies o% light, bt trans"arent to others& Jlass is o"a+e to ltra-iolet light,

bt trans"arent to -isible light&

=rigin of Light

:cientists today acce"t the eistence o% "hotons and their weird wa-e!"article

beha-ior& hat they still debate is the more eistential side o% things, sch as where
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light came %rom in the >rst "lace& o answer this +estion, "hysicists trn their

attention to the big bang and the %ew moments that %ollowed&

o might recall that the big bang is the birthing e-ent that ga-e rise to the

ni-erse& o can read more in @ow the ig ang heory orks, bt it will be se%l

to remind yo o% the basics here& *bot 15 billion years ago, all matter and energywere bottled " in a small region known as a singularity& In an instant, this single

"oint o% s"er!dense material began to e"and at an incredibly ra"id rate& *s the

newborn ni-erse e"anded, it began to cool down and become less dense& his

allowed more stable "articles and "hotons to %orm&

@ere(s what may ha-e ha""ened#

1& Immediately a%ter the big bang, electromagnetism didn(t eist as an

inde"endent %orce& Instead, it was oined to the weak nclear %orce&

2& Carticles known as and bosons also eisted at this time&

3& hen the ni-erse was st 0&00000000001 seconds old, it had cooled

enogh %or electromagnetism to break %rom the weak nclear %orce and %or

the and bosons to combine into "hotons& he "hotons mingled %reely

with +arks, the smallest bilding blocks o% matter&

4& hen the ni-erse was 0&00001 seconds old, +arks combined to %orm

"rotons and netrons&

5& hen the ni-erse was 0&01 seconds old, "rotons and netrons began to

organi?e into atoms&

6& Dinally, when the ni-erse was the tender age o% 380,000 years old, "hotons

broke %ree, and light streamed across the dark chasms o% s"ace&

his light e-entally dimmed and reddened ntil, >nally, the nclear %rnaces in

stars kicked on and began generating new light& /r sn trned on abot 4&6 billion

years ago, showering the solar system with "hotons& hose "hotons ha-e been

streaming to or hmble ble "lanet e-er since& * %ew %ell on the eyes o% great

thinkers !! Bewton, @ygens, instein!! and cased them to sto", to think and to

imagine&

How &olar 6ells Work
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o(-e "robably seen calclators with solar cells !! de-ices that ne-er

need batteriesand in some cases, don(t e-en ha-e an o) btton& *s long as there(s

enogh light, they seem to work %ore-er& o may also ha-e seen larger solar

"anels, "erha"s on emergency road signs, call boes, boys and e-en in "arking

lots to "ower the lights&

*lthogh these larger "anels aren(t as common as solar!"owered calclators,

they(re ot there and not that hard to s"ot i% yo know where to look& In

%act, photovoltaics!! which were once sed almost eclsi-ely in s"ace, "oweringsatellites( electrical systems as %ar back as 1958 !! are being sed more and more in

less eotic ways& he technology contines to "o" " in new de-ices all the time,

%rom snglasses to electric -ehicle charging stations&

he ho"e %or a .solar re-oltion. has been $oating arond %or decades !! the idea

that one day we(ll all se %ree electricity %rom the sn& his is a sedcti-e "romise,

becase on a bright, snny day, the sn(s rays gi-e o) a""roimately 1,000 watts o%

energy "er s+are meter o% the "lanet(s sr%ace& I% we cold collect all o% that

energy, we cold easily "ower or homes and oHces %or %ree&

In this article, we will eamine solar cells to learn how they con-ert the sn(s energydirectly into electricity& In the "rocess, yo will learn why we(re getting closer to

sing the sn(s energy on a daily basis, and why we still ha-e more research to do

be%ore the "rocess becomes cost!e)ecti-e&

Chotovoltaic 6ellsE 6onverting Chotons to %lectrons

he solar cells that yo see on calclators and satellitesare also called "hoto-oltaic

C cells, which as the name im"lies "hoto meaning .light. and -oltaic meaning
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.electricity., con-ert snlight directly into electricity& * modle is a gro" o% cells

connected electrically and "ackaged into a %rame more commonly known as a solar

"anel, which can then be gro"ed into larger solar arrays, like the one o"erating at

Bellis *ir Dorce ase in Be-ada&

Choto-oltaic cells are made o% s"ecial materials called semicondctors sch assilicon, which is crrently sed most commonly& asically, when light strikes the cell,

a certain "ortion o% it is absorbed within the semicondctor material& his means

that the energy o% the absorbed light is trans%erred to the semicondctor& he

energy knocks electrons loose, allowing them to $ow %reely&

C cells also all ha-e one or more electric >eld that acts to %orce electrons %reed by

light absor"tion to $ow in a certain direction& his $ow o% electrons is a crrent, and

by "lacing metal contacts on the to" and bottom o% the C cell, we can draw that

crrent o) %or eternal se, say, to "ower a calclator& his crrent, together with

the cell(s -oltage which is a reslt o% its bilt!in electric >eld or >elds, de>nes the

"ower or wattage that the solar cell can "rodce&

hat(s the basic "rocess, bt there(s really mch more to it& /n the net "age, let(s

take a dee"er look into one eam"le o% a C cell# the single!crystal silicon cell&

How &ilicon :akes a &olar 6ell

:ilicon has some s"ecial chemical "ro"erties, es"ecially in its crystalline %orm&

*n atomo% silicon has 14 electrons, arranged in three di)erent shells& he >rst two

shells !! which hold two and eight electrons res"ecti-ely !! are com"letely %ll& he

oter shell, howe-er, is only hal% %ll with st %or electrons& * silicon atom will

always look %or ways to >ll " its last shell, and to do this, it will share electrons with

%or nearby atoms& It(s like each atom holds hands with its neighbors, ece"t that in

this case, each atom has %or hands oined to %or neighbors& hat(s what %orms

the crystalline structure, and that strctre trns ot to be im"ortant to this ty"e

o% C cell&

he only "roblem is that "re crystalline silicon is a "oor condctor o% electricity

becase none o% its electrons are %ree to mo-e abot, nlike the electrons in more

o"timm condctors like co""er& o address this isse, the silicon in a solar cell

has impurities!! other atoms "r"ose%lly mied in with the silicon atoms !! which

changes the way things work a bit& e sally think o% im"rities as something

ndesirable, bt in this case, or cell woldn(t work withot them&


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hen energyis added to "re silicon, in the %orm o% heat %or eam"le, it can case

a %ew electrons to break %ree o% their bonds and lea-e their atoms& * hole is le%t

behind in each case& hese electrons, called free carriers, then wander randomly

arond the crystalline lattice looking %or another hole to %all into and carrying an

electrical crrent& @owe-er, there are so %ew o% them in "re silicon, that they aren(t

-ery se%l&

t or im"re silicon with "hos"horos atoms mied in is a di)erent story& It takes

a lot less energy to knock loose one o% or .etra. "hos"horos electrons becase

they aren(t tied " in a bond with any neighboring atoms& *s a reslt, most o% these

electrons do break %ree, and we ha-e a lot more %ree carriers than we wold ha-e in

"re silicon& he "rocess o% adding im"rities on "r"ose is called doping, and

when do"ed with "hos"horos, the reslting silicon is called type.n. %or

negati-e becase o% the "re-alence o% %ree electrons& B!ty"e do"ed silicon is a

mch better condctor than "re silicon&

he other "art o% a ty"ical solar cell is do"ed with the element boron, which hasonly three electrons in its oter shell instead o% %or, to become C!ty"e silicon&

Instead o% ha-ing %ree electrons, Ctype.". %or "ositi-e has %ree o"enings and

carries the o""osite "ositi-e charge&

(natomy of a &olar 6ell

e%ore now, or two se"arate "ieces o% silicon were electrically netralA the

interesting "art begins when yo "t them together& hat(s becase withot

an electric 8eld, the cell woldn(t workA the >eld %orms when the B!ty"e and C!ty"e

silicon come into contact& :ddenly, the %ree electrons on the B side see all the

o"enings on the C side, and there(s a mad rsh to >ll them& o all the %ree electrons>ll all the %ree holesF Bo& I% they did, then the whole arrangement woldn(t be -ery

se%l& @owe-er, right at the>unction, they do mi and %orm something o% a barrier,

making it harder and harder %or electrons on the B side to cross o-er to the C side&

-entally, e+ilibrim is reached, and we ha-e an electric >eld se"arating the two

sides&

his electric >eld acts as a diode, allowing and e-en "shing electrons to $ow

%rom the C side to the B side, bt not the other way arond& It(s like a hill !! electrons

can easily go down the hill to the B side, bt can(t climb it to the C side&

hen light, in the %orm o% "hotons, hits or solar cell, its energy breaks a"artelectron!hole "airs& ach "hoton with enogh energy will normally %ree eactly one

electron, reslting in a %ree hole as well& I% this ha""ens close enogh to the electric

>eld, or i% %ree electron and %ree hole ha""en to wander into its range o% in$ence,

the >eld will send the electron to the B side and the hole to the C side& his cases

%rther disr"tion o% electrical netrality, and i% we "ro-ide an eternal crrent "ath,

electrons will $ow throgh the "ath to the C side to nite with holes that the electric

>eld sent there, doing work %or s along the way& he electron $ow "ro-ides
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the current, and the cell(s electric >eld cases a voltage& ith both crrent and

-oltage, we ha-e power, which is the "rodct o% the two&

here are a %ew more com"onents le%t be%ore we can really se or cell& :ilicon

ha""ens to be a -ery shiny material, which can send "hotons boncing away be%ore

they(-e done their ob, so

an antire9ective coatingis a""lied to redce those losses& he >nal ste" is to

install something that will "rotect the cell %rom the elements !! o%ten a glass cover

plate& C modles are generally made by connecting se-eral indi-idal cells

together to achie-e se%l le-els o% -oltage and crrent, and "tting them in a

strdy %rame com"lete with "ositi-e and negati-e terminals&

@ow mch snlight energy does or C cell absorbF n%ortnately, "robably not an

aw%l lot& In 2006, %or eam"le, most solar "anels only reached eHciency le-els o%

abot 12 to 18 "ercent& he most ctting!edge solar "anel system that year >nally

mscled its way o-er the indstry(s long!standing 40 "ercent barrier in solar

eHciency !! achie-ing 40&7 "ercent Ksorce# &:& e"artment o% nergyL& :o why is

it sch a challenge to make the most o% a snny dayF

)he familiar sight of a rainbow represents >ust a sliver of the greater

electromagnetic spectrum.

%nergy Loss in a &olar 6ell

isible light is only "art o% the electromagnetic s"ectrm& lectromagnetic radiationis not monochromatic !! it(s made " o% a range o% di)erent wa-elengths, and

there%ore energy le-els& :ee @ow 'ight orks%or a good discssion o% the

electromagnetic s"ectrm&

'ight can be se"arated into di)erent wa-elengths, which we can see in the %orm o%

a rainbow& :ince the light that hits or cell has "hoton so% a wide range o% energies,

it trns ot that some o% them won(t ha-e enogh energy to alter an electron!hole
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"air& hey(ll sim"ly "ass throgh the cell as i% it were trans"arent& :till other "hotons

ha-e too mch energy& /nly a certain amont o% energy, measred in electron -olts

e and de>ned by or cell material abot 1&1 e %or crystalline silicon, is re+ired

to knock an electron loose& e call this the band gap energyo% a material& I% a

"hoton has more energy than the re+ired amont, then the etra energy is lost&

hat is, nless a "hoton has twice the re+ired energy, and can create more thanone electron!hole "air, bt this e)ect is not signi>cant& hese two e)ects alone can

accont %or the loss o% abot 70 "ercent o% the radiation energy incident on or cell&

hy can(t we choose a material with a really low band ga", so we can se more o%

the "hotonsF n%ortnately, or band ga" also determines the strength -oltage o%

or electric >eld, and i% it(s too low, then what we make " in etra crrent by

absorbing more "hotons, we lose by ha-ing a small -oltage& Gemember

that "oweris -oltage times crrent& he o"timal band ga", balancing these two

e)ects, is arond ".; eJ%or a cell made %rom a single material&

e ha-e other losses as well& /r electrons ha-e to $ow %rom one side o% the cell tothe other throgh an eternal circit& e can co-er the bottom with a metal,

allowing %or good condction, bt i% we com"letely co-er the to", then "hotons can(t

get throgh the o"a+e condctor and we lose all o% or crrent in some cells,

trans"arent condctors are sed on the to" sr%ace, bt not in all& I% we "t or

contacts only at the sides o% or cell, then the electrons ha-e to tra-el an etremely

long distance to reach the contacts& Gemember, silicon is a semicondctor!! it(s not

nearly as good as a metal %or trans"orting crrent& Its internal resistance

called series resistance is %airly high, and high resistance means high losses& o

minimi?e these losses, cells are ty"ically co-ered by a metallic contact grid that

shortens the distance that electrons ha-e to tra-el while co-ering only a small "art

o% the cell sr%ace& -en so, some "hotons are blocked by the grid, which can(t be

too small or else its own resistance will be too high&

&olarpowering a House

hat wold yo ha-e to do to "oweryor hose with solar energyF *lthogh it(s not

as sim"le as st sla""ing some modles on yor roo%, it(s not etremely diHclt to

do, either&

Dirst o% all, not e-ery roo% has the correct orientation or angle of inclinationto

take %ll ad-antage o% the sn(s energy& Bon!tracking C systems in the Borthern

@emis"here shold ideally "oint toward tre soth, althogh orientations that %acein more easterly and westerly directions can work too, albeit by sacri>cing -arying

degrees o% eHciency& :olar "anels shold also be inclined at an angle as close to the

area(s latitde as "ossible to absorb the maimm amont o% energy year!rond& *

di)erent orientation andMor inclination cold be sed i% yo want to maimi?e

energy "rodction %or the morning or a%ternoon, andMor the smmer or winter& /%

corse, the modles shold ne-er be shaded by nearby trees or bildings, no
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matter the time o% day or the time o% year& In a C modle, i% e-en st one o% its

cells is shaded, "ower "rodction can be signi>cantly redced&

I% yo ha-e a hose with an nshaded, sothward!%acing roo%, yo need to decide

what si?e system yo need& his is com"licated by the %acts that

yor electricity"rodction de"ends on the weather, which is ne-er com"letely"redictable, and that yor electricity demand will also -ary& 'ckily, these hrdles

are %airly easy to clear& =eteorological data gi-es a-erage monthly snlight le-els

%or di)erent geogra"hical areas& his takes into accont rain%all and clody days, as

well as altitde, hmidityand other more sbtle %actors& o shold design %or the

worst month, so that yo(ll ha-e enogh electricity year!rond& ith that data and

yor a-erage hosehold demand yor tility bill con-eniently lets yo know how

mch energy yo se e-ery month, there are sim"le methods yo can se to

determine st how many C modles yo(ll need& o(ll also need to decide on a

system -oltage, which yo can control by deciding how many modles to wire in

series&

&olving &olar Cower Issues

he thoght o% li-ing at the whim o% the weatherman "robably doesn(t thrill most

"eo"le, bt three main o"tions can ensre yo still ha-e "ower e-en i% the sn isn(t

coo"erating& I% yo want to li-e com"letely o) the grid, bt don(t trst yor C

"anels to s""ly all the electricity yo(ll need in a "inch, yo can se a back"

generator when solar s""lies rn low& he second stand!alone system in-ol-es

energy storage in the %orm o% batteries& n%ortnately, batteries can add a lot o%

cost and maintenance to a C system, bt it(s crrently a necessity i% yo want to

be com"letely inde"endent&

he alternati-e is to connect yor hose to the tility grid, bying "ower when yo

need it and selling it back when yo "rodce more than yo se& his way, the

tility acts as a "ractically in>nite storage system& Nee" in mind thogh,

go-ernment reglations -ary de"ending on location and are sbect to change& or

local tility com"any may or may not be re+ired to "artici"ate, and the byback

"rice can -ary greatly& o(ll also "robably need s"ecial e+i"ment to make sre the

"ower yo(re looking to sell the tility com"any is com"atible with their own& :a%ety

is an isse as well& he tility has to make sre that i% there(s a "ower otage in

yor neighborhood, yor C system won(t contine to %eed electricity into "ower

lines that a lineman will think are dead& his is a dangeros sitation

called islanding, bt it can be a-oided with an anti!islanding in-erter !! something

we(ll get to on the net "age&

I% yo decide to se batteries instead, kee" in mind that they(ll ha-e to be

maintained, and then re"laced a%ter a certain nmber o% years& =ost solar "anels

tend to last abot 30 years and im"ro-ed longe-ity is certainly one research goal,

bt batteries st don(t ha-e that kind o% se%l li%e Ksorce# Bational Genewable
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nergy 'aboratoryL& atteries in C systems can also be -ery dangeros becase o%

the energy they store and the acidic electrolytes they contain, so yo(ll need a well!

-entilated, nonmetallic enclosre %or them&

*lthogh se-eral di)erent kinds o% batteries are commonly sed, the one

characteristic they shold all ha-e in common is that they are deepcyclebatteries& nlike yor car battery, which is a shallow!cycle battery, dee"!cycle

batteries can discharge more o% their stored energy while still maintaining long li%e&


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he other "roblem besides energy storageis that the electricitygenerated by yor

solar "anels, and etracted %rom yor batteries i% yo choose to se them, is not in

the %orm that(s s""lied by yor tility or sed by the electrical a""liances in yor

hose& he electricity generated by a solar system is direct crrent, so yo(ll need

an inverterto con-ert it into alternating crrent& *nd like we discssed on the last

"age, a"art %rom switching < to *ne!tning new ways to make

solar "ower increasingly com"etiti-e with traditional energy sorces&

Dor eam"le, single!crystal silicon isn(t the only material sed in C cells&

Colycrystalline silicon is sed in an attem"t to ct man%actring costs, althogh the
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reslting cells aren(t as eHcient as single crystal silicon& :econd!generation solar

cell technology consists o% what(s known as thin8lm solar cells& hile they also

tend to sacri>ce some eHciency, they(re sim"ler and chea"er to "rodce !! and they

become more eHcient all the time& hin!>lm solar cells can be made %rom a -ariety

o% materials, inclding amor"hos silicon which has no crystalline strctre,

gallim arsenide, co""er indim diselenide and cadmim tellride&

*nother strategy %or increasing eHciency is to se two or more layers o% di)erent

materials with di)erent band ga"s& Gemember that de"ending on the sbstance,

"hotons o% -arying energies are absorbed& :o by stacking higher band ga" material

on the sr%ace to absorb high!energy "hotons while allowing lower!energy "hotons

to be absorbed by the lower band ga" material beneath, mch higher eHciencies

can reslt& :ch cells, called multi>unction cells, can ha-e more than one electric

>eld&

6oncentrating photovoltaic technologyis another "romising >eld o%

de-elo"ment& Instead o% sim"ly collecting and con-erting a "ortion o% whate-ersnlight st ha""ens to shine down and be con-erted into electricity, concentrating

C systems se the addition o% o"tical e+i"ment like lenses and mirrors to %ocs

greater amonts o% solar energy onto highly eHcient solar cells& *lthogh these

systems are generally "ricier to man%actre, they ha-e a nmber o% ad-antages

o-er con-entional solar "anel set"s and encorage %rther research and

de-elo"ment e)orts&

*ll these di)erent -ersions o% solar cell technology ha-e com"anies dreaming "

a""lications and "rodcts that rn the gamt, %rom solar "owered "lanes and

s"ace!based "ower stations to more e-eryday items like C!"owered crtains,

clothes and la"to" cases& Bot e-en the miniatre world o% nano"articles is being le%tot, and researchers are e-en e"loring the "otential %or organically "rodced solar

cells&

t i% "hoto-oltaic are sch a wonder%l sorce o% %ree energy, then why doesn(t the

whole world rn on solar "owerF

&olar Cower 6osts

:ome "eo"le ha-e a $awed conce"t o% solar energy& hile it(s tre that snlightis

%ree, the electricitygenerated by C systems is not& here are lots o% %actors

in-ol-ed in determining whether installing a C system is worth the "rice&

Dirst, there(s the +estion o% where yo reside& Ceo"le li-ing in snny "arts o% the

world start ot with a greater ad-antage than those settled in less sn!drenched

locations, since their C systems are generally able to generate more electricity& he

cost o% tilities in an area shold be %actored in on to" o% that& lectricity rates -ary

greatly %rom "lace to "lace, so someone li-ing %arther north may still want to

consider going solar i% their rates are "articlarly high&
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Bet, there(s the installation costA as yo "robably noticed %rom or discssion o% a

hosehold C system, +ite a bit o% hardware is needed& *s o% 2009, a residential

solar "anel set" a-eraged somewhere between O8 and O10 "er watt to install

Ksorce# Bational Genewable nergy 'aboratoryL& he larger the system, the less it

ty"ically costs "er watt& It(s also im"ortant to remember that many solar "ower

systems don(t com"letely co-er the electricity load 100 "ercent o% the time&dent that C will one day be cost!e)ecti-e in rban areas as well as remote

ones& Cart o% the "roblem is that man%actring needs to be done on a large scale to

redce costs as mch as "ossible& hat kind o% demand %or C, howe-er, won(t eist

ntil "rices %all to com"etiti-e le-els& It(s a catch!22& -en so, as demand and

modle eHciencies rise constantly, "rices %all, and the world becomes increasingly

aware o% the en-ironmental concerns associated with con-entional "ower sorces,

it(s likely "hoto-oltaics will ha-e a "romising %tre&

How (ir 6onditioners Work

he >rst modern air conditioning system was de-elo"ed in 1902 by a yong

electrical engineer named illis @a-iland


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time&


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we(ll look at how the di)erent "arts o% an air conditioner work to make all that

"ossible&

)he Carts of an (ir 6onditioner

'et(s get some hosekee"ing to"ics ot o% the way be%ore we tackle the ni+e

com"onents that make " a standard air conditioner& he biggest ob an air

conditioner has to do is to cool the indoor air& hat(s not all it does, thogh& *ir

conditioners monitor and reglate the air tem"eratre -ia a thermostat& hey also

ha-e an on board >lter that remo-es airborne "articlates %rom the circlating air&*ir conditioners %nction as dehmidi>ers& ecase tem"eratre is a key com"onent

o% relati-e hmidity, redcing the tem"eratre o% a -olme o% hmid air cases it to

release a "ortion o% its moistre& hat(s why there are drains and moistre!collecting

"ans near or attached to air conditioners, and why air conditioners discharge water

when they o"erate on hmid days&

:till, the maor "arts o% an air conditioner manage re%rigerant and mo-e air in two

directions# indoors and otside#

%vaporator Gecei-es the li+id re%rigerant

6ondenser Dacilitates heat trans%er

%2pansion valve reglates re%rigerant $ow into the e-a"orator

6ompressor * "m" that "ressri?es re%rigerant

he cold side o% an air conditioner contains the e-a"orator and a %an that blows air

o-er the chilled coils and into the room& he hot side contains the com"ressor,
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condenser and another %an to -ent hot air coming o) the com"ressed re%rigerant to

the otdoors& In between the two sets o% coils, there(s an&

e2pansion valve& It reglates the amont o% com"ressed li+id re%rigerant mo-ing

into the e-a"orator& /nce in the e-a"orator, the re%rigerant e"eriences a "ressre

dro", e"ands and changes back into a gas&

)hecompressoris actally a large electric "m" that "ressri?es the re%rigerant

gas as "art o% the "rocess o% trning it back into a li+id& here are some additional

sensors, timers and -al-es, bt the e-a"orator, com"ressor, condenser and

e"ansion -al-e are the main com"onents o% an air conditioner&

*lthogh this is a con-entional set" %or an air conditioner, there are a co"le o%

-ariations yo shold know abot& indow air conditioners ha-e all these

com"onents monted into a relati-ely small metal bo that installs into a window

o"ening& he hot air -ents %rom the back o% the nit, while the condenser coils and a

%an cool and re!circlate indoor air& igger air conditioners work a little di)erently#


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* window air conditioner nit im"lements a com"lete air conditioner in a small

s"ace& he nits are made small enogh to >t into a standard window %rame& o

close the window down on the nit, "lg it in and trn it on to get cool air& I% yo

take the co-er o) o% an n"lgged window nit, yo(ll >nd that it contains#

* com"ressor

*n e"ansion -al-e

* hot coil on the otside

* chilled coil on the inside

wo %ans

* control nit

he %ans blow air o-er the coils to im"ro-e their ability to dissi"ate heat to the

otside air and cold to the room being cooled&

hen yo get into larger air!conditioning a""lications, its time to start looking at

s"lit!system nits& * s"lit!system air conditioner s"lits the hot side %rom the cold

side o% the system, as in the diagram below&

he cold side, consisting o% the e"ansion -al-e and the cold coil, is generally

"laced into a %rnaceor some other air handler& he air handler blows air throgh

the coil and rotes the air throghot the bilding sing a series o% dcts& he hot

side, known as the condensing nit, li-es otside the bilding&

he nit consists o% a long, s"iral coil sha"ed like a cylinder& Inside the coil is a %an,to blow air throgh the coil, along with a weather!resistant com"ressor and some

control logic& his a""roach has e-ol-ed o-er the years becase it(s low!cost, and

also becase it normally reslts in redced noise inside the hose at the e"ense o%

increased noise otside the hose& /ther than the %act that the hot and cold sides

are s"lit a"art and the ca"acity is higher making the coils and com"ressor larger,

there(s no di)erence between a s"lit!system and a window air conditioner&

In warehoses, large bsiness oHces, malls, big de"artment stores and other

si?eable bildings, the condensing nit normally li-es on the roo% and can be +ite

massi-e& *lternati-ely, there may be many smaller nits on the roo%, each attached

inside to a small air handler that cools a s"eci>c ?one in the bilding&

In larger bildings and "articlarly in mlti!story bildings, the s"lit!system

a""roach begins to rn into "roblems& ither rnning the "i"e between the

condenser and the air handler eceeds distance limitations rns that are too long

start to case lbrication diHclties in the com"ressor, or the amont o% dct work
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