Physics - Millennium Schools Ideas Part... · Physics HSC Course Stage 6 From ideas to implementation Part 2: The amazing cathode ray tube

PhysicsHSC Course

Stage 6

From ideas to implementation

Part 2: The amazing cathode ray tube

Part 2: The amazing cathode ray tube 1

Contents

Introduction ............................................................................... 2

Cathode ray tubes (CRT) .......................................................... 3

Types of cathode ray tube ..................................................................3

What makes up a CRT? ......................................................................4

Cathode ray oscilloscope (CRO).........................................................5

Using the CRO .....................................................................................6

Cathode ray tubes in television sets........................................ 11

Safety requirements ..........................................................................11

Inside the picture tube .......................................................................12

The picture .........................................................................................14

Controlling the cathode ray beam ....................................................19

Electron microscopes ............................................................. 22

Types of electron microscopes .........................................................22

Lightning conductors .............................................................. 27

Photocopiers .......................................................................... 29

Summary................................................................................. 31

Suggested answers................................................................. 33

Exercises – Part 2 ................................................................... 35

2 From ideas to implementation

Introduction

In the last part of this module you learned about the history of the

development of the cathode ray tube and its role as an instrument of

scientific research into the nature of subatomic particles, namely the

electron. In this part you will learn about some of the applications for the

cathode ray tube. Many of these applications directly affect daily life

and have improved our understanding of the world around us.

In Part 2 you will be given opportunities to learn to:

• outline the role in a cathode ray tube of:

– electrodes in the electron gun

– the electric field

– the fluorescent screen

• outline applications of cathode rays in oscilloscopes, electronmicroscopes and television sets

• discuss the impact of increased understanding of cathode rays andthe development of the oscilloscope on experimental physics.

In Part 2 you will be given opportunities to:

• gather, analyse and process information on the use of electrically

charged plates and point charges in photocopying machines and

lightning conductors

• gather secondary information to identify the use of magnetic fields in

television sets.

Extracts from Physics Stage 6 Syllabus © Board of Studies NSW, originally

issued 1999. The most up-to-date version can be found on the Board's website

at http://www.boardofstudies.nsw.edu.au/syllabus99/syllabus2000_list.html


Cathode ray tubes (CRT)

The German scientist Karl Braun invented the cathode ray tube with a

fluorescent screen making up one end of the tube in 1897. It rapidly

became a research tool for scientist such as Thomson. Braun discovered

that a stream of electrons would make a screen coated with fluorescent

material glow with light.

Braun was the developer of the cathode ray oscilloscope. He

demonstrated the first oscilloscope tube in 1897, after research work on

high frequency alternating currents.

Cathode ray tubes previous to Braun's work had produced uncontrolled

cathode ray streams. Braun succeeded in producing a narrow stream of

electrons directed by means of alternating voltages that could trace

patterns on a fluorescent screen.

Types of cathode ray tube

CRTs are divided into two major groups based on how the electron beam

(or beams) are directed to the desired location on the tube's screen.

• CRTs where the electron beam is deflected electrostatically or by

electric fields. This system of electron beam deflection is used

primarily for oscilloscopes where great speed is required to position

the electron beam to a desired location on the screen in order to

follow rapidly changing waveforms.

• CRTs where the electron beam is electromagnetically deflected.

This type of CRT is used almost universally in television, computer

displays and radar.

The figure following shows a computer monitor CRT. Notice the copper

electromagnetic deflection coils.


Computer monitor cathode ray picture tube. Photo: Ric Morante.

Color CRTs are similar to other electromagnetically deflected CRTs

except that they contain the equivalent of three conventional electron

guns in one envelope along with a more complex screen configuration in

terms of the distribution of the phosphors into discrete areas in the

fluorescing screen.

What makes up a CRT?

The basic components of all modern CRTs are outlined below.

• An electron beam source and beam intensity control mechanism.

• One or more accelerating electrodes. These increase the electron’s

velocity so that when the electrons hit the screen they have enough

energy to ensure appropriate light output from the screen. The faster

the electrons are travelling, the brighter the fluorescence produced

by the screen.

• A focusing section consisting of electric fields that bring the electron

beam to a sharp focus precisely at the screen. Most CRTs have a

curved front screen so that the focusing mechanism required by the

television produces a focused beam at the same distance from the

filament, no matter where on the screen the electron beam hits.


• A deflection system consisting of magnetic coils that positions the

beam to a desired location on the screen or is used to scan the beam

across and down the screen in a repetitive pattern.

• A phosphor screen that converts the kinetic energy of the electrons

in the invisible electron or cathode ray beam into visible light.

• A mechanical structure known as the envelope or tube outer that

allows a vacuum state (eg. the glass tube). This must also provide a

location for electrical connections to the various electrodes, and must

insulate those connections and components from each other.

Why does the CRT screen glow?

The screen glows because of phosphorescence. Cathode ray tubes and

television picture tubes have a layer of the phosphor coated on the inside

of the glass screen. The beam of electrons projected by the cathode

excites the fluorescent phosphor layer on the screen. That is, the

phosphors glow.

Most cathode ray tubes use zinc sulfide that glows with a characteristic

blue-green trace. In a colour TV tube, the screen is coated with three

phosphors that fluoresce – one red, another green and the third blue.

With phosphorescence, the emission of light from a phosphorescent

substance can continue for a brief time after the exciting radiation of the

electron beam is cut off. That is why when the TV is finally turned off at

night the screen appears to have a soft glow in the darkened room for a

short time. This is because the phosphor material absorbs energy from

earlier exposure to the beam of electrons which is stored and gradually

re-emitted later as light.

Phosphorescent materials include zinc, calcium, barium and strontium

sulfides.

Cathode ray oscilloscope (CRO)

A cathode ray tube that is configured as a cathode ray oscilloscope is

shown in the figure following.


1 kV potential difference

cathode

electron

grid

vertical deflection plates

horizontal deflection plates

electrons in a beam

vacuum

a bright spot is formed at thepoint where the electron beammeets this luminescent screen

filament

fluorescent screen

The internal 3-D view of a modified cathode ray tube that makes up a CRO.The deflection plates are set with positive or negative potentials with an ACcurrent. The changing electric fields across the plates can cause rapiddeflection of the electrons in the electron beam.

The stream of electrons from the cathode can be focussed to a point at the

end of the tube on a fluorescent screen by use of a device called an

electron gun. The electron gun consists of:

· a filament that is the source of electrons

· a negatively charged metal cylinder that focuses the electron beam in

much the same way as a condensing (convex) lens focuses light

· a series of metal anode cylinders that have progressively higher

positive potentials (voltages) relative to the cathode.

The beam is focussed to a dot by the use of electric fields from the anode

cylinders. The effect of the electric fields from a two anode electron gun

are illustrated in the figure following.

The invention of the cathode ray oscilloscope was the forerunner of the

television picture tube and radarscope. It also became an important

laboratory research instrument.

Braun's career in science came to an end when he travelled to New York

City in 1915 to testify in a radio related patent case. He was detained

there because of his German citizenship when the U.S. entered World

War I in 1917. He died in 1918 before the war ended. He apparently did

not contribute to the allied war effort while in detention and so was loyal

to his native Germany.


F

anode 1

anode 2

equipotential lines

electron beam

screen is at thebeam focusmore positive

positive

negatively charged cylinderacts as a condensing lensto narrow the beam

electrons are emittedat the filament

An electron gun. The anodes shown in the figure are each part of a cut awaycylinder. These cylinders enable the electron beam to be focussed in crosssection to a cylindrical beam.

1 Outline the role of the electron gun in the cathode ray tube.

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2 What does the cylindrical electric plates in the anodes do to the

electron beam?

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3 Why is the fluorescing screen so important in the modern cathode

ray tube used in applications such as the CRO or television?

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Check your answers.


Using the CRO

You are already familiar with the use of the CRO to study sound waves

in the module The world communicates. This use of the CRO is not the

typical one made in research. Rather the CRO is used as a research tool

wherever low, fluctuating voltages occur. In the electronics industry that

application has guided development of the communication age in which

you currently live. This means the CRO has been used in the

development of just about every communication and computing device

known.

Locate a local electronics enthusiast or someone who professionally repairs

electronic appliances such as televisions or computers. Ask them how they

use the CRO in their work. Write down their responses in the space below.

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How does the CRO work?

The voltage to the horizontal deflection plates is adjusted so as to move

the electron beam across the screen at a constant rate. The vertical

deflection plates are subject to varying or constantly varying AC

voltages. This moves the electron beam irregularly in the vertical plane.

The combined motion produces a trace on the phosphor screen.

This is illustrated in the following where a sine wave trace is produced

on the screen of the CRO by a constant AC voltage input.


zero voltage across the verticaldeflection plates

a constant DC voltage acrossthe vertical deflection plates

dot moves up (ordown) the screen

dot in the centre ofthe screen

a constant AC current across thevertical deflection platesproduces a line

spot oscillates vertically

spot moves horizontallyat steady speed

A combination of a constanthorizontal deflection and aAC deflection onthe vertical plates producesa sine curve

The trace from a CRO produced by an AC current.


The trace shown above is that produced by a regular AC voltage.

The pale text at the bottom indicates that each division on the grid in the

y-direction represents 0.25 V. The time base is shown as 5.005 ms per

division of the x-direction grid.

1 Look at the figure on the previous page. What does this indicate about

the rate of movement of the electron beam across the screen in the

x-direction?

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2 What movement of the deflection plates would produce that

movement you described above? Which deflection plates in the

CRO tube would be involved?

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3 What movement of the electron beam is necessary in the y-direction

to produce the pattern shown?

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4 How can this y-direction movement of the electron beam be

produced by the deflection plates in the CRO tube?

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Check your answers.

Do Exercises 2.1 to 2.2 now.


Cathode ray tubes in television sets

Traditional television sets contain a modified cathode ray tube as a

picture tube. The picture tube is a funnel shaped cathode ray tube made

of special glass. The glass tubes must be able to seal to the metal

electrodes that carry high voltages (the anode buttons) and to the

conductors supplying the heating power and control voltages to the

cathode. Together these form part of the electron beam gun.

electron gun

magneticdeflection

coils

silvered tubefluorescent screen

A television cathode ray picture tube.

Safety requirements

The glass used to manufacture the picture tube has specific requirements

for safety reasons. The television picture tube is essentially a particle

accelerator. Electrons are accelerated from the cathode toward the anode

and eventually the phosphor coated screen. Accelerating an electron

causes it to give out electromagnetic radiation. That happens in the

cathode ray picture tube of a television or computer monitor. The result

is often that low levels of high energy electromagnetic radiation such as

X-rays are produced.


The electrons accelerated toward the screen in the picture tube are

undergoing an electrical energy (E = qV) loss and a kinetic energy gain12

2mvÊË

ˆ¯ .

People are exposed to the radiation from the television picture tubes for

extended periods of time. Because of this the glass used in the tube must

contain appropriate amounts of heavy oxides such as barium oxide, lead

oxide and strontium oxide in order to ensure potentially harmful X-rays

produced by the tube in its operation are absorbed.

Survey the number of hours the television set in your home is on per week.

Do this by simply recording the time(s) that the television is turned on and

off each day for a week. Many dwellings have the television operating for

up to 120 hours per week.

Sun Mon Tues Wed Thurs Fri Sat

AM

PM

Total hours

Consider that low levels of X-rays can be produced by television picture

tubes. The dosage of X-rays that can lead to health problems is

cumulative. If the body is exposed to excess levels of X-ray radiation it

can cause problems such as cancer.

Can you see why it is important to prepare the glass tubes that make up

the television picture tube from glass that absorbs X-rays?

Inside the picture tube

In a cathode ray picture tube, the cathode is a heated filament similar to

the filament in a normal light bulb. The cathode filament is heated in a

vacuum created inside a glass funnel shaped tube. As a result, electrons

naturally escape from the heated cathode into the vacuum of the tube.

The free electrons are negative. The anode is positive, so it attracts the

electrons given off from the cathode and accelerates them towards it.

The accelerated electrons actually pass through the hole in the anode,

overshoot and continue on a path toward the other end of the tube that

makes up the television screen.


In a television’s cathode ray picture tube, the accelerating stream of

electrons (or cathode ray) is focused into a narrow beam by a set of

focusing anodes. This arrangement is again called an electron gun.

The electron gun is located at the rear of the glass discharge tube called thepicture tube. The electron gun makes the cathode ray beam that is eventuallyresponsible for painting the picture on the fluorescing screen.(Photo: Ric Morante)

The cathode ray beam flies through the vacuum in the tube until it hits

the screen at the other end of the tube. This screen is really just the end

of a cathode ray tube coated with phosphor. This phosphor coating is

concentrated in small dots or rectangles on the screen. The phosphor

glows when struck by the beam. Rapid movement of the electron beam

across and down the screen at the end of the tube produces the images

you see on the television.

If you look at the labelled photograph of the cathode ray picture tube

above you will see there is a conductive silver coating inside the tube.

This conductive coating is there to soak up the electrons that pile up at

the screen-end of the tube. Hence if you look at the inside a broken

picture tube you will see it is silver coated.


The picture

How is a single beam of electrons able to produce a picture on the television

screen ?

The viewing of a picture on a television screen relies on some quirks of how

your brain works. The first of these is that if you divide a still image into a

collection of small dots, your brain will reassemble the dots into an image.

Look closely at a photograph in a newspaper. You should be able to

distinguish the picture as being made up of a series of dots. If you have

difficulty doing this, use a magnifying glass or convex lens. This can be

made with a water drop on a sheet of clear plastic or cellophane

wrapping.

1 What colour are all the dots that make up the image?

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Now look closely at a colour image from a magazine either with a

magnifying glass or a convex water lens on a sheet of clear plastic.

2 How many colours are all the dots that make up the image?

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As you can see making a picture from dots is easy. Now check a

television for evidence of a similar technology for making images.

Carry out the following activity.

Turn your television on. Flick a few drops of water onto your television

screen. Look closely at the small convex lenses formed by the water

droplets.

3 Describe what you see.

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Check your answers.

Picture resolution

If you have access to a computer with a television style monitor rather

than a plasma or LCD screen and can put a multicoloured image on the

screen repeat the experiment with the water droplets described above.

You should see identical results although you will probably see the dots

or rectangles are closer together than on a television screen.


This produces a higher resolution on the computer screen. The still

image allows you to examine various colours of the dots that make up the

screen colour in a particular part of the screen. You should see the dot

colour combinations vary in relative intensities. Mixing the intensities of

the different coloured dots enables the eye to construct all the colours

that can be seen on a screen.

You may be familiar with the term, dpi in printing. This refers to the

number of dots per inch. The more dots per inch the higher the

resolution of the image. A similar thing happens with computer and

television screens. The more dots per inch, the higher the resolution.

Each of the different coloured dots you see in a rectangle, stripe or dot is

made to fluoresce by an electron beam specifically aimed at that portion

of the dot. In other words there is an electron beam that is aimed only at

the red dots, another only aimed at the blue dots and a third aimed only at

the green dots. This means whereas a black and white television has a

single electron beam a colour television picture tube needs three beams.

How does the image form?

You should now know that the image formed on a television screen is

made up of a series of dots. The question you should now ask yourself is

how does a single electron beam on a white phosphor screen produce an

image in black and white, or how does three electron beams produce a

coloured image? The answer lies in another quirk of the eye.

This quirk is one you are already familiar with and probably experience

everyday. When you look at a bright light such as a light globe filament

the image of that bright object is clearly impressed on the eye for some

time even after you stop looking at the bright object. This is even more

apparent if you close your eyes after looking at the bright object. You

will still see a residual image of the bright object apparently impressed

on your eyelids.

Another example of this happening is if you spin a torch or bright light

around in a circular motion on a dark night. The passage of the torch

forms a circular ring pattern on the retina of the eye. This creates the

appearance of a ring of light even though the torch can only be in one

place at any one time.


Moving pictures

The final quirk of the eye that allows you to see moving picture is as

follows.

If you photograph a moving object a number of times in close

succession, a sequence of still pictures is produced. If you show the still

pictures in rapid succession, your brain reassembles the sequence of still

pictures back into a moving scene.

You don’t have to be special to do this. In fact, cartoons and all

animations are based on this concept. You may even have tried to

produce your own animation using something as simple as stick figures

on the corners of the pages of a book that you can flick through rapidly to

give the stick figures an appearance of motion. If you have never tried

this you should have a go now with the corners of the page of the

learning materials.

Watching motion

If you have access to a video cassette recorder (VCR) and a tape try the

following activity.

1 Put a recorded video in your VCR.

2 Press play.

3 Press pause and advance the video by the frame advance if the VCR has

that option.

Can you see the individual frames that are assembled and played one

after the other to produce the effect of a moving image from still frames

taken close together? If the frames are flicking through and changing

fast enough your brain and eyes do not seen any of these individual

frames. You see a moving picture scene.

A clear picture

The bright spot left by the moving electron beam in the cathode ray tube

moves across and down the screen many times per second (50 times for

most televisions but can be higher for example 100 times or 100 Hz for

flicker free televisions). Everywhere it touches the phosphor screen the

screen fluoresces. The result is a picture produced on the retina of your

eye of the pattern arced out by the bright spot on the screen.

This picture appears to be on the screen of the cathode ray picture tube

even though in reality each electron beam is only causing illumination at

one point on the screen at any one instant of time.


To assist this process there is also a slight lag time where the

fluorescence of the screen is maintained even though the electron beam

moves on. However, the length of this lag time glow must be short to

produce a clear picture and avoid interference.

Instead of deflection plates, steering coils produce the movement of the

electron beam within the cathode ray tube. Steering coils are copper

windings around the outside of the tube. These current carrying coils

create magnetic fields inside the tube when an electric current passes

through them. The electron beam is deflected by the magnetic fields.

One set of steering coils creates a magnetic field that pushes the electron

beam down the screen (the vertical deflection plates of the electron gun).

Another set of steering coils creates the magnetic fields that push the

beam horizontally (the horizontal deflection plates of the electron gun).

A computer monitor showing deflection coils. (Photo: Ric Morante.)

By controlling the size and direction of the currents flowing in the coils

you can position the electron beam to any point on the screen.

In a television picture tube, to paint the entire screen and trick the eye

into seeing this illumination as a constantly repainted moving picture, the

magnetic field generated by the current carrying coils to need to move

the electron beam in a ‘raster scan’ pattern across and down the screen.

In other words, the beam paints one line across the screen from left to

right.


These lines are rows of dots of different brightness varying with the

intensity of the electron beam. At the end of a line the beam is switched

off then quickly turned back on when aimed at the left hand side of the

screen again but has been pushed down screen slightly. It then ‘paints’

another horizontal line and the whole process is repeated. This continues

down the screen until the entire screen has been painted. The electron

beam is then switched off until it is aimed at the top left hand corner

again ready to begin repainting the screen once more. The whole screen

is painted 50 times per second to produce the effect of a moving picture

scene.

If the television is a 100 Hz model the process described above is

repeated 100 times per second. The more times per second the screen is

painted, the greater the flicker free effect of the picture. Manufacturers

of televisions claim this flicker free television is less demanding on the

eyes and promotes more comfortable viewing. These 100 Hz televisions

are not as common as 50 Hz televisions because of their higher cost.

Computer monitors that work in almost the identical manner are often

designed so that the screen is painted in excess of 50 times per second to

produce higher resolution flicker free images. Some computer monitors

allow you to change the refresh or repaint rate of the monitor to suit your

own requirements up to around 100 Hz. Interestingly most video cassette

machines send the signal to the television for repainting the picture only

25 times per second yet the picture is relatively flawless and appears

continuous.

1 Seek out a 50 Hz television, a 100 Hz television, a television playing a

video tape and a high resolution computer monitor. This may be most

easily accomplished by visiting an electrical goods shop.

Compare the image in terms of viewing quality. Write down your

impressions as to whether you can tell the difference or see the

flickering of the image.

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2 Watch television until you see a shot of a working television on the

screen. This often happens during the news. Observe the image of

the screen of the television on your screen. Do you notice the

definite flicker?

Alternatively if you have access to a video camera take a video of

the screen of an operating television. Replay the video though your

television. You should see the image of the screen flicker.


Why is this flickering of the television screen image visible to you much

more prominently in the video footage even though if you were watching the

television you wouldn't notice the flicker?

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Check your answer.

Controlling the cathode ray beam

In the module The world communicates you learned that a signal is

necessary to produce a response in the electronic communication devices.

When a television aerial receives a broadcast radio signal, or when you

use a video cassette recorder (VCR) or digital video disc player (DVD) to

play a program on your television, the signal has to contain the

information to control the electron beams in the cathode ray picture tube.

That is the signal needs to contain the information that tells the

electronics controlling the beams of electrons hitting the phosphor screen

how to behave. This is so that the image on the phosphor screen is

accurate to the one the TV station, DVD or VCR sends.

The TV station or VCR therefore sends a signal to the TV that contains

four different parts.

• Intensity signals for the electron beam as it paints each line. This

controls the brightness of the glow on the phosphor screen to enable

shading on the image.

• Horizontal retrace signals the TV when to move the electron beam

very quickly back at the end of each line.

• Vertical retrace signals to move the beam from bottom right to top

left in conjunction with the horizontal.

• A signal indicates which of the electron beams aimed at the coloured

phosphor dots on the screen should be turned on or off to illuminate

phosphors necessary to make the colour required in that part of the

screen.

A signal that contains all of these components is called a composite

video signal. Of course that signal will only give you the picture. The

sound is produced by the decoding of a simultaneously transmitted FM

audio signal! Note that both signals are radio frequencies.


Magnets and the screen

To do the following activity you will need access to a television monitor and

a magnet. Any magnet will do, even a strong fridge magnet.

Take care when doing this activity. It is possible with some older

televisions to permanently damage the screen. Never bring the magnet close

enough to touch the screen. Do not leave the magnet in place near the

screen for more than a second or so.

Procedure

• Turn on the television.

• Bring the magnet close to the television screen and move it around

near the screen but do not allow the magnet to come into contact

with the screen. If you do it could damage your screen permanently.

1 Describe what you see happening to the image on the screen.

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2 How does this suggest that the image produced on the television

screen is produced as a result of a beam of electrons?

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Check your answers.

In times past, television screens could be dramatically affected by

magnetic fields from common devices such as the magnetic field

associated with a vacuum cleaner motor. To eliminate this screen image

problem it was often necessary to get a TV repairman to degauss the

screen to restore a damaged image.

Now it is common for TV screens to have a slightly greenish tinge in

parts. This is often found to be due to having some source of a magnetic

field too close to the screen. Possible sources of magnetic fields include

external speakers or poorly designed additional electrical appliances.


It is now common for computer monitors to have a built in adjustment

that allows you to degauss the screen should it be affected by a stray

magnetic field by accident. Other devices such as televisions are

degaussed when you switch the television of and on.

Locate a TV repairman and ask about the role of magnetic fields in

producing a clear picture on the television screen.


Electron microscopes

Almost everyone has looked at photographs taken by an electron

microscope. Usually these photographs show exquisite detail of very

small objects. What most people do not realise is that the electron

microscope is really behaving as a giant cathode ray tube. The main

difference between an electron microscope and a light microscope is the

way that the device forms images. The electron microscope uses a

stream of electrons instead of a stream of light photons to form the

image. So, electron microscopes are really just scientific instruments that

use a beam of highly energetic electrons to examine objects on a very

fine scale.

Electron microscopes were developed to overcome the magnification andresolution limitations of the light microscopes. Light microscopes are

limited by the physics of light to 500X or 1000X magnification and have

a resolution limit of 0.2 mm.

The problem is that resolution is determined by the wavelength of theradiation being employed.

By the early 1930s these limits had been achieved. The desire to seefiner detail was expanding with biologists seeking to see the interior

structures of cells and the structure of the cell membrane. To see moredetail required resolution and magnification at least ten times greater than

could be supplied by the light microscope.

The size of electron vibrations is tiny compared to the wavelength of

light. This means a greater resolving power for the electron microscope.The aim was, therefore, to build an electron microscope.

Types of electron microscopes

There are two basic types of electron microscopes: the transmission

electron microscope (TEM) and the scanning electron microscope

(SEM).


The transmission electron microscope (TEM) was the first type of

electron microscope to be developed. It was developed by Max Knoll

and Ernst Ruska in Germany in 1931. Its mode of operation was

patterned on the light transmission microscope you are most probably

familiar with. The difference is that a focused beam of electrons was

used to 'see' through the specimen rather than light.

The first experimental scanning electron microscope (SEM) operated in

1942 but the first commercial instruments didn't become available until

around 1965. A scanning electron microscope works in much the same

way as a light reflection microscope except that instead of the image

forming from reflected light the image forms from reflected electrons.

All electron microscopes work in basically the same way.

1 A stream of electrons is formed by a cathode filament and

accelerated toward and through an anode toward the specimen in a

vacuum tube.

2 The electron stream or cathode ray is focused using metal apertures

and electromagnetic lenses into a thin, focused beam. The metal

apertures simply stop any stray electrons from outside the main

beam interfering with the production of a clear image. The role of

these devices is similar to the electron gun in a television or CRO.

3 The cathode ray beam is focused onto the sample using an

electromagnetic lens. In general the beam scans across and down the

specimen.

4 Interactions occur between the electrons and the specimen. These

directly affect the electron beam transmission or reflection and it is

this alteration in the electron beam that is detected and transformed

into an image on a phosphor screen.

The transmission electron microscope

This type of microscope is generally used to examine biological

specimens. The microscope is essentially a sealed vacuum tube. Heating

a tungsten filament at voltages usually at around 500 V produces the

cathode or electron source. Then the electrons are accelerated at voltages

generally ranging from 60 000 to 100 000 V. This high voltage

accelerates the electrons toward the anode with the result that the

electrons pass through the specimen. For the electrons to pass through

the sample it must be cut very thin.

Because electron beams are invisible to the eye, the images they form are

revealed on a fluorescent phosphor screen that is essentially a high

resolution black and white television screen and can then be

photographed.


To increase the contrast in the thin sample it is stained with electron

absorbing heavy metal salts that are preferentially absorbed in some parts

of the specimen. The sample is loaded into a chamber at normal air

pressure. The chamber is attached to a vacuum pump and the whole

system is evacuated before the electron beam can begin to ‘look’ at the

sample.

filament acting as a cathode

magnetic field

electromagnets acting asa condenser lens system

system under vacuum

magnetic fields refocus thebeamand act as an objectivelens

sample cut thinly

electron beam hits aphosphor screen andforms a magnifiedimage just like animage on a televisionscreen

phosphor screen

electron beam thatpases through thethin sample

the person viewsthe light imagethat forms on thephosphor screen

light image from thephosphor screen

condenser ringaperture tocollect strayelectrons

projector lens magnetic fields

The transmission electron microscope.

The first lens largely determines the ‘spot size’ – the general size range

of the final spot that strikes the sample. The second lens changes the size

of the spot on the sample; changing it from a wide dispersed spot to a

pinpoint beam.

A condenser ring is used to prevent any stray electrons that are not part

of the tight beam hitting the object and producing a fuzzy halo on the

image. This also restricts the electron beam.

The tightly focussed beam strikes the specimen. Some of the beam is

directly transmitted, while some is absorbed or scattered. The transmitted

portion is focused by the objective lens to form an image that is passed

down the tube through intermediate and projector lenses. These lenses

have the role of enlarging the image before it forms on the phosphor

screen.


At the phosphor screen the energy of the moving electrons is converted

into light. This allows the user to see the image or photograph it.

Darker areas of the image represent those that fewer electrons were

transmitted through because they were thicker or denser. This is aided

by the contrast enhancing chemicals mentioned earlier.

Lighter areas of the image represent areas of the sample that more

electrons were transmitted through because they were thinner or less

dense or absorbed less by less of the electron absorbing salts.

The scanning electron microscope

This type of microscope is also a large modified cathode ray tube under

near vacuum conditions.

filament source of electrons

condenser lens magneticfields focus the cathoderay beam

electromagnet

electromagnet producinga magnetic field to deflectelectron beam scans thesample that has beencoated with gold

beam of electrons

an image of the object isproduced line by line as ona normal television butfaithful to the informationcollected from the reflectedelectron beam

vacuum chamber

information fromthe detector isrelayed to acathode ray tubemonitor

detector collects thescattered and reflectedelectrons

reflected electronsfrom sample

the path of theelectron beamis from side toside and downthe sample

the gold coatedsample reflectselectrons from thebeam to the detector

condenser ringaperture tocollect strayelectrons

Scanning electron microscope.


At the top of the tube is the electron gun producing a stream of electrons.

The stream is condensed into a narrow beam by the condenser lenses that

use a series of electromagnetic coils. This lens system works in

conjunction with a condenser aperture to eliminate any stray electrons

not focussed into the beam. The objective lens focuses the scanning

beam onto the specimen.

The specimen is gold coated to allow it to conduct any charge build up

away rapidly. If this is not done small delicate specimens can be affected

by the electrostatic repulsion from the beam. They can be blown away

from the beam used to investigate them. If the specimen is valuable or

unique this can be a serious problem. The specimen is usually coated

with a fine coat of gold in a ‘sputtering machine’.

The electron beam is set to scan across the specimen in a similar raster

pattern to the pattern used in the television. When the beam strikes the

sample, interactions between the electrons in the beam and sample occur

and are detected. These interactions are usually reflections. Before the

beam moves on, the number of interactions detected are counted and

display as a pixel or dot on a CRT phosphor screen as light energy. The

number and nature of electron interactions determine the intensity of a

pixel on the screen. More interactions produce brighter pixels. The

scanning process continues until the entire grid representing the sample is

scanned. The grid scan is repeated up to 30 times per second. The entire

process is outlined on the figure above.

Do Exercises 2.3 to 2.6 now.


Lightning conductors

Lightning protection systems are designed to prevent damage to people

and property due to a large electrical discharge of static electricity that

has built up in clouds. The discharges can be of the order of 200 000 A.

The diagram below shows a typical lightning protection system on a

domestic dwelling.

lightning rods

conductingcopper wire

metal strapgrounding plates andground termination rods

A domestic dwelling with a lightning protection system installed.

The main purpose of the system is to conduct the electrical charge of a

lightning strike safely away from property and people along a designated

path. Hopefully, the system intercepts and guides the electric current

harmlessly to ground.

A typical domestic lightning protection system is made up of several

components:

• Air terminals or lightning rods: These are usually slender metal rods

that are installed on the roof at regular intervals. In the past these

rods were always pointed though new research suggests that blunt

rods may be better suited as lightning rods.


• Conductors: These are usually copper cables that connect the air

terminals and the other system components.

• Ground terminations: These are metal rods driven into the earth to

guide the lightning current harmlessly to ground where it will

dissipate. These rods may be attached to a metal sheet or plate to

make the dissipation of the electric current more efficient.

In large buildings or installations of a commercial nature, such as

communications dishes, a conductive strap of metal may be used to

ensure that the current is spread evenly over a number of ground

terminations. These ground terminations are often buried up to 3 m

below the surface in trenches. This reduces the danger to people and

equipment that may be close to the ground termination when there is

a lightning strike.


Photocopiers

Most photocopiers receive their information one page at a time and print

using electrostatic charges, toner and laser light. The actual way

individual copiers work varies a lot. The description that follows is an

outline of a possible pathway by which a photocopier may operate.

Photocopying involves the application of electrostatic charges and heat to

produce copies of all kinds of written, printed and graphic matter. The

basis of the photocopying process is photoconductivity.

Photoconductivity is the ability of certain substances to allow an electric

current to flow through them when struck by light. The chemical

element, selenium is a photoconductive material. It is a poor electrical

conductor in low light levels but, when light energy is absorbed by some

of the selenium’s electrons and a voltage is applied, the conductivity of

the selenium increases. This occurs because electrons are able to pass

more freely from one atom to another after absorbing the light energy.

When the light source is removed, the electron mobility falls and the

selenium becomes a non-conductor again.

A photocopier uses an aluminum drum coated with a layer of selenium.

The light from the document to be copied is reflected from its surface to

the selenium surface of the copier drum.

Particles of toner are sprayed through a nozzle, gain a negative charge

and stick to the drum in the dark areas where no reflection from the

original occurred. This is the print or picture. In doing so this forms an

image of the document on the drum that is reversed as when viewed in a

mirror.

A sheet of copy paper is passed close to the rotating drum. A positive

electrically charged plate under the paper causes the negatively charged

toner to move across to the copy paper. The toner is then sealed to the

paper by a hot roller that fuses the toner particles to the paper. Voila, a

copy!


The process of making a copy in the machine is as follows.

• The selenium coated drum is cleaned of any excess toner from the

surface with a rubber blade as it rotates.

• The drum is given a negative charge of about 600 V.

• A light beam passes over the paper copy from one end to the other in

a band. The reflection increases the conductivity in the lighted areas

of the drum. This causes portions of the drum not illuminated by

reflected light (dark areas of the original) to become more positively

charged.

• Toner particles made negative so they act as point charges as they

pass out of their cartridge are applied to the rotating drum and are

attracted to the areas of positive charge.

The negative areas of the drum that have been illuminated repel the

negatively charged toner particles. The surface of the drum at this

point is acting as a rotating charged plate.

As the selenium coated drum rotates out of the illuminated area it

loses its conductive and positive charged areas. This means the

toner in contact with the drum is held only weakly to the surface

because there is no longer a strong electrostatic attraction.

• The paper is fed by rollers under the rotating drum where a large

positively charged plate attracts the negatively charged toner from

the drum surface to the paper.

The drum turns as the paper runs beneath it so the toner mirror

image is transferred from the drum to the paper as a replica of the

original. The paper runs through the fusing roller that is heated to

about 200°C. This fuses the toner onto the paper.

Do Exercise 2.7 now.

If you have a access to a photocopier open the front cover and look at the

drum, the positive plate where the paper picks up the toner, the fusing roller

and the toner cartridge. Do not touch, look only, and have someone who

knows about photocopiers show you the different parts.


Summary

Electrodes in the electron gun are used to: _______________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Electric fields in cathode ray tubes: ____________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

The fluorescent screen in a cathode ray:_________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

The most common use for cathode ray tubes is in the:______________

_________________________________________________________

Oscilloscopes are sensitive to: ________________________________

_________________________________________________________

_________________________________________________________

Oscilloscopes have applications such as: ________________________

_________________________________________________________

_________________________________________________________


A photocopier works by: _____________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________


Suggested answers

Cathode ray oscilloscope (CRO)1 The electron gun produces a focused cathode ray beam that is

accelerated toward the fluorescent screen using a system of electric

fields. The electron beam is aimed at different sections of the screen

using either electric or magnetic fields.

2 The electric cylindrical plates attract the beam toward the screen

giving the beam particles kinetic energy and a rapid response time

when the beam must be moved on to new spot.

3 The screen converts the energy of the electron beam into light

energy that is easy to detect with the human eye.

How does the CRO work?1 The rate of movement in the x-direction is at a constant rate until it

snaps back.

2 Horizontal deflection plates provided a constant electric field. When

the end of the display is reached the beam must switch off and

repaint the screen.

3 The movement of the electron beam is varying in the up and down

direction.

4 The y-direction movement is produced by the vertical deflection

plates given a varying voltage.

Inside the picture tube1 The dots are all black. By varying their density in any area an image

can be built-up.

2 You will see that what appears to be a solid colour is actually made

up of separate dots of colour. The dots are all one of four colours.

Those colours are cyan, magenta, yellow, blue and black.


3 You should see small rectangles or circles. If the television is a

coloured television you should see these dots are red, blue or green.

If the television is a black and white television you should see the

dots are only black or white.

A clear picture

The sequencing of the refresh rates on the screen are not synchronous so

the image is only captured occasionally (relatively speaking) by the video

tape. The result is a longer than expected blank screen. If this is long

enough it appears as an annoying flicker on the taped TV monitor.

Magnets and the screen1 The image seems to distort. The screen may get a greenish tinge.

The image appears to be deflected or distorted by the effect of the

magnetic field.

2 Electrons are deflected by a magnetic field. The distortion of the

screen image suggests that whatever is producing the image is

deflected by a magnetic field.


Exercises – Part 2

Exercises 2.1. to 2.7 Name: _________________________________

Exercise 2.1

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Exercise 2.2

Outline the sequence of events that led to the development of the CRO

with particular reference to the development of an increased

understanding of cathode rays.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Exercise 2.3

The cathode ray tube in an oscilloscope, television and electron

microscope have a number of features in common. List the common

features. Describe the common role each feature has in the operation of

the device.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________


Exercise 2.4

Describe how the magnetic fields used in television sets control the

production of the image.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

Exercise 2.5

What is the role of the electron gun in a television?

_________________________________________________________

_________________________________________________________

_________________________________________________________

Exercise 2.6

An image can be created on the screen of a black and white television

even though the size of the dot produced by the electron beam at any

instant of time is at a maximum the size of a pin head.

Explain how this is possible by referring to the characteristics of the

screen and the human eye. Discuss the role of magnetic fields in making

the image possible in your answer.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________


Exercise 2.7

The transfer of an image from the drum of a photocopier to the paper is

an example of use of electrostatic charges on extremely fine powder

particles of toner and charged plates. Outline the main features of that

process that involve the use of electrostatics.

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

_________________________________________________________

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Physics - Millennium Schools Ideas Part... · Physics HSC Course Stage 6 From ideas to implementation Part 2: The amazing cathode ray tube