LIGHT& the Electromagnetic Spectrum

Examining Light 2008

• We will investigate the following:• 1. What is light?• 2.What are some sources of light around us?• 3. What are opaque, transparent, & translucent objects?• 4. What is a light wave?

Light

• Our primary source of light is the sun.• Light travels in straight lines at a speed of

186,000miles per second.• *Light waves travel faster than sound waves!• Light energy from the sun travels through space ,

reaches earth, and some of it turns to heat energy and warms the earth’s air.

• Light from the sun also travels to the cells of green plants (producers) and is stored as energy.

• When light reaches an object, it is absorbed, reflected, or passes through it.

Sensing Light• Humans have two

light detectors.• Do you know what

they are called?

How many sources of Light can you list and explain?

• SUN=warms air, water, and land.• Fire=provides heat, light,

and cooking fuel.• Lightning=• Firefly=• Flashlight=• Light bulb=• Laser beams=• Optical telephone fibers=*Traffic lights=

*AIMS: Primarily Physics:Light Sources Activity

“Just Passing Through:” What happens when light strikes

glass? Or waxed paper? Or a book?

• If light travels through an object it is =transparent

• If light is blocked by an object and a dark shadow is cast it is= opaque.

• If some light passes through but not all and a light shadow is present it is=translucent.

*AIMS:Primarily Physics: Just Passing Through Activity

What happens when light hits these objects?

• Glass of water • School bus window• Notebook paper• Waxed paper• Plastic wrap• Tissue paper• Cardboard• Textbook• Hand lens…

Transparent objects:

• The windows on a school bus,• A clear empty glass,• A clear window pane,• The lenses of some eyeglasses,• Clear plastic wrap,• The glass on a clock,• A hand lens,• Colored glass…• ALL of these are transparent.

Yes, we can see through them because light passes through each of them.

Translucent objects• Thin tissue paper,• Waxed paper,• Tinted car windows,• Frosted glass,• Clouds,• All of these materials

are translucent and allow some light to pass but the light cannot be clearly seen through.

Opaque objects:

• Heavy weight paper,• Cardboard• Aluminum foil,• Mirror, bricks, buildings,• Your eyelids and hands,• Solid wood door,• All of these objects are

opaque because light cannot pass through them at all.

• They cast a dark shadow.

What is light really?Electromagnetic radiation waves

• Light waves are three dimensional.

• Light waves vibrate in all planes around a center line.

• The waves have high points called “crests.”

• Waves also have low points called “troughs.”

• *The distance from one crest to the next crest is called a “wavelength.”

• *The number of waves passing a given point in one second is called the “frequency.”

wavelength

*A Science Museum of VA:Light Science Activity

Notice the wavelength is long(Radio waves) and gets shorter (Gamma Rays)

*Electromagnetic Radiation

• Electromagnetic radiation can be described as a stream of photons. Each photon traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy.

• The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons. Radio waves have photons with low energies. Microwaves have a little more energy than radio waves. Gamma-rays and cosmic rays have highest energy waves and are the deadliest.

*Page info from NSTA Conference 2004

*Page info from NSAT Conference 2004

Don’t’ forget…longest waves (radio) ..to shortest waves (cosmic)

*Science Museum of VirginiaActing It Out! Activity +Scaling the Spectrum Activity ORGEMS: Invisible UniverseComparing Wave Makers I.

Radio(Longest electromagnetic waves)

• Emitted by– Astronomical Objects– Radio Station

Transmitters

• Detected by– Ground based radio

telescopes– *If you turn on a radio,, it

will convert the radio wave energy into sound energy.

Television• Shorter than radio,

also used to carry messages (pictures & sound) to our TV sets.

• *We can sense the TV waves around us with our televisions.

Microwave

• Emitted by:– Gas clouds collapsing

into stars– Microwave Ovens– Radar Stations– Cell Phones

• Detected by– Microwave Telescopes– Food (heated)– Cell phones– Radar (systems)

Infrared(Heat or Thermal)

Are you a source of infrared? YES you are!

• Emitted by– Sun and stars (Near)– TV Remote Controls– Food Warming Lights

(Thermal)– *Everything at room

temperature or above,=HEAT

• Detected by– Infrared Cameras– TVs, VCRs,– Your skin

Let’s take a look at Herschel’s Experiment

• Herschel’s Experiment– Discovered Invisible

Light– In 1800, Herschel

places his control thermometer just outside the red end of the spectrum

– Result: The outside thermometer registered the highest temperature

Let’s set up Herschel’s Experiment

• Set up of Box Design for Conducting the Herschel Experiment.

Conducting Hershel’s Experiment• Place a sheet of white paper

inside a cardboard box• Tape three thermometers

together and place inside box

• Cut a small notch in the top of the box and position a glass prism so that the spectrum is projected inside the box

• Arrange the thermometers so that one is just outside the red end of the spectrum, with no visible light falling on it

VisibleEach color is a different size wave.

Red the longest & violet the shortest• Emitted by

– The sun and other astronomical objects

– Laser pointers– Light bulbs

• Detected by– Cameras

(film or digital)– Human eyes– Plants (red light)– Telescopes

Color

• White light is not a single color; it is made up of a mixture of the seven colors of the rainbow.

We can demonstrate this by splitting white light with a prism:

This is how rainbows are formed: sunlight is “split up” by raindrops.

The colors of the rainbow:

• Red

• Orange

• Yellow

• Green

• Blue

• Indigo

• Violet

Adding colors• White light can be split up to make separate colors.

These colours can be added together again.

• The primary colors of light are red, blue and green:

Adding blue and red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white

again

Adding red and green makes yellow

Seeing color• The colour an object appears depends on the colours

of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is reflected

A white hat would reflect all seven colours:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):

Purple light

White

light

Using colored light

• If we look at a colored object in coloured light we see something different. For example, consider a football kit:

White

light

Shorts look blue

Shirt looks red

• In different colors of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Some further examples:

Object Colour of lightColour object seems to be

Red socks

Red Red

Blue Black

Green Black

Blue teddy

Red Black

Blue

Green

Green camel

Red

Blue

Green

Magenta book

Red

Blue

Green

Using filters• Filters can be used to “block” out different colors of light:

Red Filter

Magenta Filter

Investigating filters

Colour of filter Colours that could be “seen”

Red

Green

Blue

Cyan

Magenta

Yellow

Red

Magenta

White

Yellow

Blue Green

Cyan

UltravioletSunburn / black light

• Emitted by– Tanning booths (A)– The sun (A)– Black light bulbs (B)– UV lamps

• Detected by– Space based UV

detectors– UV Cameras– Flying insects (flies)

X-ray• Emitted by

– Astronomical objects – X-ray machines– CAT scan machines– Older televisions– Radioactive minerals– Airport luggage

scanners

• Detected by– Space based X-ray

detectors– X-ray film– CCD detectors

Chandra X-ray Observatory

• Chandra is designed to observe X-rays from high energy regions of the universe, such as the remnants of exploded stars.

• The most sophisticated observatory built to date.

• Deployed by the Space Shuttle Columbia on July 23, 1999,

Chandra X-ray Observatory

*Slide from 2004 NSTA Conference

Gamma Ray(Short electromagnetic waves but more energetic)

• Emitted by– Radioactive materials– Exploding nuclear

weapons– Gamma-ray bursts– Solar flares

• Detected by• --Geiger counters

– Gamma detectors and astronomical satellites

– Medical imaging detectors

Sources of g-ray Emission• Black holes• Active Galaxies• Pulsars• Diffuse emission• Supernovae• Gamma-ray bursts• Unidentified

COSMIC Rays(The highest energy waves and the deadliest)

• Cosmic rays come from deep space and can pass through the Earth.

• A great question!• Radio waves= (Buildings to human size);• Microwaves

(Humans-beetles);• Infrared waves (Eye of a needle);Visible waves (microscopic size)!

WOW! All the rest are the size of molecules, atoms, atomic nuclei and smaller..

Snell’s Law !

or

Reflection• Reflection from a mirror:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

• The Law of ReflectionThe Law of Reflection

Angle of incidence = Angle of reflectionAngle of incidence = Angle of reflection

In other words, light gets reflected from a surface at ____ _____ angle it hits it.

The same !!!

Clear vs. Diffuse Reflection

• Smooth, shiny surfaces have a clear reflection:

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

Using mirrors

• Two examples:

1) A periscope

2) A car headlight

RefractionLight bends towards

the normal in a denser material

Refraction happens because the light slows down in the

material

(the part of the beam that arrives first slows down first)

Glass

Air

You can experiment with effects of different incident angles and refractive indices with Figure 3. You can move the flashlight around, and change the refractive index of the top and bottom mediums. You can use the buttons to quickly enter the index for a few common mediums. Critical AngleUsing the refraction simulator, notice how the light bends toward the normal when the light enters a medium of greater refractive index, and away from the normal when entering a medium of lesser refractive index. Then notice what happens when you move the flashlight to an angle close to 90 or -90 degrees in the medium with a higher refractive index. As you approach the critical angle the refracted light approaches 90 or -90 degrees and, at the critical angle, the angle of refractions becomes 90 or -90 and the light is no longer transmitted across the medium/medium interface. For angles greater in absolute value than the critical angle, all the light is reflected. This is called total reflection.