The Electromagnetic Spectrum
Light, Sound and Waves

Spectra and colour

for 14-16

Experiments in this collection begin with Newton's experiments, showing that white light comprises the full spectrum of colours. One experiment hints that the spectrum continues beyond the visible. Line spectra, in sunlight and from other light sources, become visible using gratings. Finally, there are experiments that introduce additive and subtractive colour mixing.

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A simple spectrum

The Electromagnetic Spectrum
Light, Sound and Waves

A simple spectrum

Practical Activity for 14-16

Class practical

Producing a simple spectrum by dispersion of light.

Apparatus and Materials

  • Ray box (see below)
  • +7D cylindrical lens
  • Single slit
  • Multiple slit
  • L.T. variable voltage supply
  • 60-degree prism
  • Pieces of coloured filter

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Direct a single streak of light at the prism and observe the emerging ray carefully. Twist the prism and observe the effect on the emerging ray.
  2. Direct a fan of rays (from lamp and multiple slit) at a +7D cylindrical lens so that the emerging rays pass through an image-point 40 to 50 cm away. Place the prism just beyond the lens and look at the effect. If you hold a small piece of paper or card upright to catch the rays above the table, the spectrum will show clearly.
  3. Try placing small pieces of colour filter in the path of the light, before it strikes the prism.

Teaching Notes

  • This should be as simple as possible, without complicated optics, as an introductory experiment.
  • The spectrum will be the most pure if the screen is held at the same optical distance from the lens as the image was before the prism was inserted, and the prism is turned to minimum deviation. However, turning the prism to a greater deviation will show a wider spectrum.
  • Different shaped prisms could be used, and the way rays pass through them compared. Looking along the ray towards the source will show that the ray appears to be straight.
  • Using a coloured filter next to the ray box, the emergent ray will also be coloured, and as the prism is rotated the colours will become clearer.

This experiment was safety-tested in January 2007

Resources

Download a copy of a document listing ray box suppliers.

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Demonstration spectrum

The Electromagnetic Spectrum
Light, Sound and Waves

Demonstration spectrum

Practical Activity for 14-16

Demonstration

Class demonstration of a spectrum, and the appearance of coloured objects in coloured light.

Apparatus and Materials

  • High-dispersion prism (flint or lead glass)
  • Convex lens
  • Lens holder
  • Light source, compact
  • Power supply for light source, low voltage, variable
  • Retort stands and bosses, 2
  • Pieces of coloured filter

Health & Safety and Technical Notes

Treat the compact light source with respect as it is a significant source of UV; do not look directly at the bulb. The lens will effectively filter out the UV.

Read our standard health & safety guidance


Note that no slit is needed if the source is compact or if it is a line filament, parallel to the prism's edge. It will be advantageous to reduce the light level in the lab.

If you don't have a compact light source (quartz iodine lamp), use a 36 W 12 V lamp overrun to 14 or 16 V.

Procedure

  1. Set up the light source at one end of the laboratory so as to produce an intense line of light. Place the lens so that a sharp image is produced on a distant screen about 3 m away. Place the glass prism in the beam, just beyond the lens, and rotate it to show how the prism swings the rays round. Move the screen so that a suitable spectrum is produced; this need not be at minimum deviation.
  2. Look at the spectrum through a selection of coloured filters. The effect of the primary and secondary coloured filters on the white light spectrum should be noted. Look at a selection of coloured materials - a sheet of stamps for example - and hold them in different parts of the spectrum.

Teaching Notes

  • Many prisms used in school are made of low dispersion glass in order to reduce the colours on the emergent ray. Here a high dispersion prism is needed, because the dispersion is what is required. The prism should be large enough to make use of the whole of the incident beam from the bright light source.
  • The position of minimum deviation produces a bright, clear spectrum without a white band in the middle. However this is not the point of maximum dispersion, and so a compromise has to be made between clarity and size of the spectrum. (To find the position of minimum deviation, rotate the prism until the spectrum rotation stops and begins to retrace its path as the prism continues to rotate in its initial direction.)
  • This spectrum is a series of images of the lamp filament in a progression of colours side by side. The screen should be slanted to spread out the colours even further. The blue light is refracted more than the red light. The wider the filament, the more overlapping there is and the less pure the spectrum, but the brighter it is.
  • Demonstrations of the recombination of coloured light to form white light are easy to set up. The prism version is less fiddly than the mirrors, but not so impressive. Remember that there is little blue light in a tungsten filament lamp, and so distance travelled by the blue light in particular should be as short as possible.
  • The reverse prism: If the lab has a second prism of the same dispersion as the one used for the spectrum demonstration, place it in the demonstration arrangement, next to the first prism but the other way round.
  • Reassembly by mirrors: Form a spectrum. Place strips of plane mirror in the spectrum: one to catch the red, one to catch the orange, etc. Twist each mirror so that all the reflected beams fall at the same place on a white screen.
  • Newton was delighted, and his contemporaries amazed, when he analyzed a beam of sunlight with a glass prism. Let your students share those feelings. Without any complicated optical system, let a streak of sunlight fall onto a high dispersion prism and then travel to a white wall of a darkened room. The sun's disc subtends an angle of 1/2° therefore each colour of the spectrum will make at least 1/2° patch in the spectrum at any distance.
  • Ask students what Newton, a very good scientist, did as his next experiment. The splitting up of light into colours was a delight, and it must have been noted by many, although they didn't think about it as Newton did. But it was an act of scientific genius to pierce a hole in the card on which he caught the spectrum, and try a second prism with the light of one colour that came through the hole: no further production of a new range of colours. That is not only sensible experimenting: it gives a strong hint that the colours were there in a mixture, waiting to be sorted once and for all.

This experiment was safety-tested in January 2007

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Investigating the spectrum with an electronic detector

The Electromagnetic Spectrum
Light, Sound and Waves

Investigating the spectrum with an electronic detector

Practical Activity for 14-16

Demonstration

A class demonstration of the spectrum.

Apparatus and Materials

  • Compact light source (quartz iodine lamp)
  • Power supply, low voltage, variable
  • Convex lens, large
  • High-dispersion prism (flint or lead glass)
  • Screen, white
  • Phototransistor
  • Cell
  • Demonstration meter (2.5-0-2.5) mA

Health & Safety and Technical Notes

Treat the compact light source with respect as it is a significant source of UV; do not look directly at the bulb. The lens will effectively filter out the UV.

Read our standard health & safety guidance


A suitable 63 mm diameter +6D plano-convex lens is obtainable from the supplier, Knight Optical, part no. LPV16063.

Connect the phototransistor in series with a 1.5-volt cell, and the demonstration meter with a 2.5-0-2.5 mA. scale. A slit may be needed to place over the phototransistor, through which the radiation can pass.

The phototransistor will probably be mounted on a board with a red and a black terminal. The red terminal should be connected to the positive side of the cell.

Note that no slit is needed if the source is compact or if it is a line filament, parallel to the prism's edge.

Procedure

  1. Position the convex lens 20 cm in front of the lamp. If a plano-convex lens is used, face the plane surface towards the lamp. An image of the hot filament will form far away. Put the high-dispersion prism after the lens and near to it. Position the white screen so that it is at the image distance from the lens, about 3 m away. The spectrum will be seen.
  2. Move the phototransistor through the visible spectrum and show the response on the meter. It will be clear that there is radiation beyond the visible end of the spectrum.

Teaching Notes

  • This demonstration shows the variation in intensity of the different colours of light from a hot filament in a glass envelope.
  • The radiation falls outside the visible region of the spectrum, indicating infra-red and ultra-violet radiation. But because the prism and the lens are made of glass, there is a sharp cut-off in the infra-red and little if any response in the ultra-violet.
  • You could extend the demonstration using a small blackened thermistor, infra-red or ultra-violet sensor, although the restriction placed by the absorption of those wavelengths by glass remains.

This experiment was safety-tested in January 2007

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Herschel’s infra-red experiment

The Electromagnetic Spectrum
Light, Sound and Waves

Herschel’s infra-red experiment

Practical Activity for 14-16

Class practical

This is a recreation of Herschel’s experiment, published in 1800, in which he discovered infra-red radiation.

Apparatus and Materials

For each student group

  • Glass prism
  • Cardboard box with open top (e.g. photocopier paper box)
  • Thermometer, 3
  • Marker pen, black
  • A4 paper, plain white, one sheet

Health & Safety and Technical Notes

Students should be warned not to look directly at the Sun.

If you use liquid-in-glass thermometers, be careful about breakages. If you use mercury–in-glass thermometers, have a small kit available for clearing up any breakages. Mercury is classed as a hazardous waste and should not be disposed of with normal waste.

Read our standard health & safety guidance


A plastic prism is unsuitable as it is likely to absorb more infra-red radiation than glass.

Procedure

  1. Blacken the thermometer bulbs with black marker ink.
  2. Cut a notch in the side of the box so that the prism will fit snugly. You need to be able to turn the prism about its long axis while it remains securely in position.
  3. Place the sheet of white paper in the bottom of the box.
  4. Out-of-doors, position the box with the prism towards the Sun. Adjust the angle of the prism to give the broadest possible spectrum on the sheet of paper. (It may help to place a book under the front edge of the box to tilt it.)
  5. Position the three thermometers so that their bulbs are in different regions of the visible spectrum (blue, yellow, red).
  6. Note the maximum temperature reached by each thermometer.
  7. Repeat, but place one of the thermometers beyond the red end of the spectrum.

Teaching Notes

  • In Herschel’s experiment, he placed only one thermometer at a time in the spectrum; the other two were on either side (where no radiation falls), to act as controls.
  • Herschel noted that the final temperature reached by his thermometers depended on the size of the bulb. This is because a large bulb has a larger area to absorb radiation, but it also has a larger area to lose radiation, and a greater mass to heat up.

How Science Works Extension: For an approach to this experiment including questions and teaching points, see: William Herschel and the discovery of infra-red radiation

You will find further teaching ideas in the relevant section of the Cool Cosmos website.:

Cool Cosmos


Acknowledgement: This experiment has been adapted from the Cool Cosmos website.

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Spectra formed by gratings

The Electromagnetic Spectrum
Light, Sound and Waves

Spectra formed by gratings

Practical Activity for 14-16

Demonstration

Observing the spectra from a variety of light sources.

Apparatus and Materials

For each student or group of students:

  • Fine diffraction grating (about 300 lines/mm)
  • Light source, compact
  • Neon lamp
  • Hydrogen discharge lamp
  • Sodium lamp or Bunsen burner and salt
  • Clear linear filament striplight in a mounting without diffuser, wired with 13 A plug
  • Appropriate power supplies for lamps

Health & Safety and Technical Notes

The UV intensity from the compact light source will be reduced by the distance between the lamp and the grating. Do not allow anyone to view the spectrum from a distance below 3m.

Read our standard health & safety guidance


The sodium source can be a lamp or salt (stick or granular) in a Bunsen flame. Alternatively, you can make a sodium flame using a filter paper. The paper is soaked in brine, folded in half and wrapped around the mixing tube of the Bunsen burner with the fold just in the flame.

Procedure

Set up the light sources at one end of the laboratory and hold the grating close to the eye. Look at the distant lamps and examine the spectra seen.

Teaching Notes

  • A white-hot filament will show a central white line where waves of all colours go straight through the grating. Out to each side is a bright band which corresponds to the first bright fringe out from the centre of Young's fringes (one wavelength path difference). Since the light is white, each bright fringe is spread into a wide spectrum of colours. Further out to each side, there may be a wider but fainter spectrum which corresponds to the next bright fringe out from the centre (two wavelengths path difference).
  • Image courtesy of Tony Reynolds
  • The neon source is a tube containing neon atoms which are being bombarded by high speed electrons. The red light comes from neon atoms as they recover from that excitation. There will be other colours too.
  • Image courtesy of Tony Reynolds
  • The hydrogen source is a tube containing hydrogen atoms which are being bombarded by high speed electrons at high voltage. As the atoms recover they give out only a few definite colours.
  • The light from a sodium source is pure yellow, and not a mixture of red and green which we accept for yellow in colour mixing. Many commercial lamps also contain neon so the spectrum also contains weak red lines.
  • Image courtesy of Tony Reynolds
  • The long filament strip light (about 25 cm) is a useful white light source because coloured filters, red, green, blue, can be wrapped around it. The displacement of the red, green or blue light will be clearly seen, showing that red light is diffracted the most.
  • An interesting extension is to record the spectra photographically. The diffraction grating is taped to a camera lens and the camera focused on the light source (the camera is replacing the eye). With coloured film loaded into the camera, or a digital camera, then a few seconds exposure will produce wonderful spectra photographs.

This experiment was safety-tested in January 2007

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Absorption spectrum of sodium

The Electromagnetic Spectrum
Light, Sound and Waves

Absorption spectrum of sodium

Practical Activity for 14-16

Demonstration

Using a spectrometer to observe the absorption lines formed when white light passes through sodium vapour.

Apparatus and Materials

  • Lamp with fine filament (12 V, 24 W)
  • Lamp holder (S.B.C.) on base
  • Power supply, low voltage, variable
  • Positive lenses (+7D), 2
  • Lens holder, 2
  • Retort stands, bosses and clamps, 2
  • Spectrometer
  • Prism (preferably high dispersion)
  • Bunsen burner with salt for sodium flame

Health & Safety and Technical Notes

Read our standard health & safety guidance


It is essential to provide an intense sodium flame.

Place the flame midway between the two lenses where the first lens will produce a sharp image. This will make all the white light pass through the flame in the region where it is rich in sodium.

Focus the spectrometer in the usual way. Keep the slit narrow.

Adjust the voltage applied to the lamp as well as the slit width to get the best conditions for seeing the dark lines.

Procedure

  1. Direct white light from the line filament lamp to the slit. Arrange the two positive lenses to make sure that all the white light entering the collimator of the spectroscope will pass through the flame on the way to it.
  2. Dip an iron wire or a ceramic rod in concentrated brine and hold it in a Bunsen flame to provide the intense sodium flame, or use the filter-paper method. See technical note for this experiment:

    Spectra formed by gratings


Teaching Notes

  • One lens forms a real image of the filament in the flame; the other lens forms a real image of that first real image on the slit of the spectroscope. It is easiest to make each of the distances, filament to lens, lens to image, image to lens and lens to slit, twice the focal length of the lens concerned.
  • The spectrum from the white light passes through cooler sodium vapour. The cooler sodium vapour absorbs energy from the white light spectrum, producing dark lines.

This experiment was safety-tested in February 2007

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Simple spectrum of sunlight

The Electromagnetic Spectrum
Light, Sound and Waves

Simple spectrum of sunlight

Practical Activity for 14-16

Class practical

Using prism, needle and sunlight to observe a spectrum.

Apparatus and Materials

For each student or group of students

  • 60-degree prism
  • Needle

Health & Safety and Technical Notes

For safety reasons, you might prefer to mount the needle in some cork.

Read our standard health & safety guidance


Procedure

  1. Hold the needle at arm's length in one hand and the prism in the other hand, close to your eye. The needle should be parallel to the refracting edge of the prism and brightly illuminated by sunlight.
  2. Twist the prism until you can see the needle by refraction through the prism.

Teaching Notes

  • The needle would appear as a bright line, but the dispersion makes it appear drawn out into a bright, white-light spectrum.
  • The higher the dispersive power of the prism material, the better.
  • It may be possible to see absorption lines, otherwise known as Fraunhofer lines. This is because the needle forms a bright, narrow line-image of the sun, which serves as a slit. The eye, viewing the needle from some distance, provides the lens system that is needed.

This experiment was safety-tested in January 2007

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Absorption lines in the spectrum of sunlight

The Electromagnetic Spectrum
Light, Sound and Waves

Absorption lines in the spectrum of sunlight

Practical Activity for 14-16

Demonstration

Using a spectrometer to see absorption lines.

Apparatus and Materials

  • Spectrometer
  • High-dispersion prism (flint or lead glass)
  • Convex lens, +6D to +10D
  • Plane mirror
  • Lens holder

Health & Safety and Technical Notes

The spectrum should be projected onto a screen rather than viewed down the telescope part of the spectrometer, to minimize the risk of stray sunlight being directed into the observer's eye.

Read our standard health & safety guidance


If the dark absorption lines do not appear, the slit needs to be made narrower. See the manufacturer's guide book for details of setting up the spectrometer.

Procedure

  1. Set up the prism spectrometer in the usual way to give a good spectrum of white light on a small screen. With the slit narrowed down very considerably, direct sunlight into the slit with a plane mirror. The lens is used to converge the light to a focus 20 to 30 mm in front of the slit.
  2. Darken the room so that the spectrum does appear to be a bright one, and ask students to view the spectrum one at a time. Arrangements have to be made to ensure that the sunlight continues to fall on the slit.
  3. Image courtesy of the Department of Physics and Astronomy at Dartmouth College, New Hampshire

Teaching Notes

  • The Sun produces an absorption spectrum, with dark lines across its spectrum. Chemical elements in the Sun's corona absorb specific wavelengths of light so their electrons are excited to higher energy levels. Emission takes place equally in every direction, with the result that the intensity of light in the Earth's direction is much reduced.
  • The dark lines are called Fraunhofer lines. These are very important in astrophysics as they reveal the composition of the outer layers of stars. Slight shifts in the positions of the lines indicate the speed of the stars' approach or recession.

This experiment was safety-tested in January 2007

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Understanding colours

The Electromagnetic Spectrum
Light, Sound and Waves

Understanding colours

Practical Activity for 14-16

Demonstration

Using filters to produce coloured lights and observe coloured objects.

Apparatus and Materials

For each student or group of students

  • Pieces of each of the following Lee plastic colour filters:
  • Primary red (No- 106)
  • Primary green (No- 139)
  • Primary blue (No- 79)
  • Magenta (purple) (No- 113)
  • Yellow (No- 101)
  • Samples of brightly coloured cloth
  • And...
  • Light source, compact
  • Power supply for light source, low voltage, variable
  • Lens, +6D
  • 60-degree prism, large (preferably high dispersion)
  • White screen or blank wall
  • Translucent screen
  • Low power lamp to go behind screen
  • Small projector to provide large patch of coloured light
  • Set of colour filters (60 mm x 60 mm) for projector
  • Bunsen burner with salt

Health & Safety and Technical Notes

If the demonstration spectrum uses the compact light source, check that the class cannot observe the white light directly.

Read our standard health & safety guidance


The Lee filter numbering system is that used most widely in the stage lighting and photographic business. Sheets of plastic filter about 1 m x 0.5 m can be obtained from photographic and lighting suppliers for less than UKP 5 per sheet (September 2006). One such supplier is: Back Stage Shop.

Pieces of filter about 100 mm x 100 mm are suitable as it is easier for students to look with both eyes: it also lessens confusion in handling if large pieces are used.

Beforehand, use a felt-tipped pen to label each filter R, G, B, C, M or Y.

It will save time and trouble if each set of six different filters is put in an envelope ready to be issued. A key list can be printed on the envelopes: e.g.

R is RED

M is MAGENTA (RED + BLUE) etc.

Procedure

Examining colour filters

  1. Set up a...

    Demonstration spectrum


    ...Arrange it so that the screen faces the class normally, but that the spectrum falls on the screen very obliquely so that a long spectrum is seen. Position the translucent screen far away from the spectrum screen and place a low wattage lamp behind it as a background to view the colour of a filter. For parts 3, 4 and 5, the rest of the room should be as dark as possible.
  2. With the lab well-lit, examine the filters, looking through each in turn at the lighted translucent screen or the bright sky. Use one filter at a time first, but allow students to experiment with combinations if they wish. Observe a piece of red cloth through a red filter and then a green filter. Repeat this using a green filter.
  3. Primary filters and spectra: Darken the room as much as possible but keep a small lamp running behind the translucent screen so that students can select the filter they want. Look at the spectrum through a red filter, then green, then blue.
  4. Secondary filters and spectra: Repeat 3 using cyan, magenta and then a yellow filter.
  5. With a small projector throw a large patch of red light on the wall by hanging a red filter on the front of the projection lens. Show pieces of coloured cloth in that red light: with no other light in the room. Change to green light (the green filter will transmit a little red as well as all green).
  6. Return to full daylight. Look at the bright screen or sky through pairs of filters in series.
  7. RED and GREEN
  8. MAGENTA AND CYAN
  9. CYAN and COMMON YELLOW
  10. MAGENTA and COMMON YELLOW
  11. Take three large pieces of filter, magenta, cyan and yellow, and tape them on a window pane, overlapping, so that they can act as a reminder to the class.

Teaching Notes

  • Some primary schools study colour so some students may remember some of this.
  • In part 2 you might ask students how the filters make light coloured. Does the red dye change all parts of the spectrum to red, or does it just cut out other colours and leave the red that was always there in the white light? Is the dye a colour adder or a colour subtractor? Filters transmit their own colour and absorb the rest. They do not dye all the light with their own colour. They subtract colour.
  • The dye in a piece of coloured cloth is a selective filter. A piece of red cloth can only return red light to our eyes and look red if it receives some red light. If red cloth is observed in white light or magenta light, its red colour can be seen because each of those contains red light. But if it is observed in just green light, the cloth looks black because it receives no red light to return.
  • The red filter is plain red.
  • The green filter is plain green and is the colour our eyes are most sensitive to.
  • The blue filter is plain blue.
  • The cyan filter is made from blue and green.
  • The magenta filter is made from red and blue.
  • The yellow filter is made from red and green and not the pure yellow of a sodium lamp.
  • In part 3 ask students what each filter does to the various coloured lights. Does a red filter blot out (absorb) parts of the spectrum or does it paint the spectrum red? Does it manufacture more red light or just leave red light that was already there? (It filters so it subtracts.)
  • Using the secondary filters in part 4 to view a white light spectrum, first of all singly, then through two or more filters, will show that secondary filters transmit two colours. Two overlapping filters will transmit only the colour common to both of them. The yellow filter will transmit red and green, as well as a little true yellow. Almost all the yellows are mainly red + green, a subjective yellow. True yellow is such a narrow band that adding it to the mixture of red and green makes little difference.
  • RED lets through only RED, GREEN lets through only GREEN. So nothing gets through both, giving BLACK.
  • CYAN + MAGENTA - only TRUE BLUE gets through
  • CYAN + YELLOW - only GREEN gets through
  • MAGENTA + YELLOW - only RED gets through
  • MAGENTA + YELLOW + CYAN - transmits nothing (BLACK)
  • So Magenta, Cyan and Yellow are best for colour printing, Technicolour films, and water-colour paints. These operate by subtractive colour mixing.
  • All colour films have three colour layers made up of each of the secondary colours. You can show this by building up a layered picture on an OHP. Look at the negative and print produced by colour film and note the colours.
  • Pure yellow comes from a narrow part of the spectrum such as the light produced in a sodium flame. A red car near a sodium street lamp will look black.
  • The dyes in filters are used in water-colour paints, and even when the paints are mixed each paint does its own filtering job. When mixing blue and yellow in a paint box to make green, you are really using cyan and yellow, and the common colour transmitted is green.

This experiment was safety-tested in January 2007

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Additive colour mixing

The Electromagnetic Spectrum
Light, Sound and Waves

Additive colour mixing

Practical Activity for 14-16

Demonstration

Mixing coloured light and observing coloured objects in that light.

Apparatus and Materials

  • Small projection lanterns (35 mm slide projectors), 3
  • Slides for projectors (see technical notes)
  • Small pieces of colour filter (6 cm x 6 cm) red, green, true blue
  • Shielded rheostats or variacs for projectors - or alternative (see technical notes), 2
  • White screen or blank wall

Health & Safety and Technical Notes

The rheostats used to control mains electricity must use proper mains plugs and sockets. It is probably easier to use cards with different apertures to restrict light output.

Read our standard health & safety guidance


The slides are squares of card or metal with a hole about 25 mm diameter. Some method of changing the brightness of one or two of the patches is necessary. A finger or piece of card held in front of the projection lens to reduce its aperture does the job nicely.

Prepare the apparatus before the class assembles. Place the slides in the projectors and focus each to make a round patch of light on the screen. Hang one filter in the front of the projection lens of each. (If the projectors are not equally bright, hang the blue filter on the brightest.) Arrange the projectors so that the three patches of coloured light overlap with a central triangle illuminated by all three. Darken the room completely. Run the blue projector at full voltage. Adjust the other two projectors (with rheostats, variacs or otherwise) until the central triangle looks white. Leave the settings fixed and turn off or cap the projectors.

Ray box kits are often sold with a version of this experiment. Slots hold red, green and blue filters on the front and sides of the box. The light is reflected by mirrors onto a screen so that the three colours overlap. Tungsten filament lamps have a lot of red light and very little blue light, and so two red filters should be put into the same slot to cut down on the intensity of the red light. The blue filter should be placed in the front slot so that it is as near to the screen as possible.

Photographs courtesy of Mike Vetterlein

Procedure

  1. Turn on the red and blue projectors. Magenta can be seen where the patches of red and blue overlap. Similarly, try blue and green; and finally red and green.
  2. Then turn on all three projectors. Wave a hand in front of the white patch, making coloured shadows.
  3. Turn on the red and blue projectors to make a large patch of magenta. Hold pieces of coloured cloth in that light: first magenta cloth, then red cloth, then green cloth. Ask for explanations.

Teaching Notes

  • Red light + green light = yellow light
  • red light + blue light = magenta light
  • green light + blue light = cyan light
  • red light + blue light + green light = white light
  • Complementary colours are two colours which add together to form white, e.g. red and cyan.
  • A red filter lets through red but subtracts green and blue. Red can therefore be called minus green and minus blue and so minus green and minus blue, is what colour printers call red.

This experiment was safety-tested in January 2007

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Mixing coloured light

Electromagnet
Light, Sound and Waves

Mixing coloured light

Practical Activity for 14-16

Demonstration

A way of demonstrating colour addition that does not require filters or lamps.

Apparatus and Materials

  • Data projector and computer
  • Small mirrors, 2 or 3
  • Prism

Health & Safety and Technical Notes

Read our standard health & safety guidance


Works best in a darkened room.

Procedure

This is a participative demonstration based on the PowerPoint presentation...

Primary Colours


...a resource that is freely available from the TES Connect website. The data projector itself acts as a high intensity light source.

Teaching Notes

  • Depending on the age and ability of students, this demonstration can take 30-50 minutes.
  • When you get to slide 4, split up a beam of white light using a prism. The spectrum looks awesome if projected onto a distant wall.
  • With slide 16, use small mirrors to reflect and combine pairs of the primary colours on a white surface. This takes some practice but it works surprisingly well. It is also possible to overlap all 3 reflected colours to show white, but this is more difficult.
  • With slide 18, ask 'If white light can be split into the full spectrum, why do we only need to combine red, blue and green light to make white?' This leads to a discussion of red, blue and green photo-sensitive cells in the eyes; the after image illusion demonstrates the effects of this.
  • At slide 24 it is worth allowing students time to write how they would design the illusions in the list, to test whether they understand the principle. Once they have decided what colours would go where,- show the working illusions.
  • Slides 21-36: Students love these illusions. Leave the colour-inverted images up for 30 seconds and ask students to stare at them. After the 30 seconds show the following white screen and ask students to blink. They should see an after image with the correct (complementary) colours.

This experiment, based on a presentation available from the website TES Connect, was submitted by Peter Tryon, Tien Shan International School.

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Spectra

The Electromagnetic Spectrum
Light, Sound and Waves

Spectra

Practical Activity for 14-16

Class practical

Students use a diffraction grating as a tool for observing the spectra from a variety of light sources.

Apparatus and Materials

  • Fine diffraction grating (about 300 lines/mm)
  • Green filter
  • Red filter
  • L.T. variable voltage supply (capable of 8 A at 12 V)
  • Lamp, 12 V 24 W
  • Lamp holder on base
  • Spectrum tube, hydrogen
  • Spectrum tube, neon
  • Spectrum tube holder, with integral power supply or 5 kV EHT power supply
  • Sodium flame

Health & Safety and Technical Notes

Where the EHT supply is used, all connections between the tube holder and the supply must be made before the supply is switched on. The tube holder should not have any exposed metal which could become live.

Read our standard health & safety guidance


Each student pair will need a fine diffraction grating, red filter and green filter.

The light sources should be mounted as high up in the laboratory as possible. The spectrum tubes will require an appropriate holder and voltage supply.

Procedure

  1. Set up the bright, white-hot filament high at the end of the laboratory.
  2. Ask students to observe the light source with the fine grating held close to the eye.
  3. Students should then look at the neon spectrum tube; at the hydrogen tube; at a slit held in front of a sodium flame; at the bright, white-hot filament, through red and green filters.

Teaching Notes

  • The capillary-tube gas-filled lamps which operate on (3-5) kV are in fact a fine-line source of light; neon tubes produce spectra consisting of many bright lines of different colours, whereas hydrogen is much fainter and produces fewer spectral lines.
  • Sodium lamps normally need to have a slit placed in front of them in order to produce a line source. Sodium light can also be produced by holding a sodium chloride stick in a Bunsen burner flame, or even by sprinkling sodium chloride into the flame, though that can be quite messy.
  • Alternatively, use the technique in this experiment:

    Interference with air wedge


  • When a tube containing neon gas, for example, 'is connected to a high voltage supply, it produces a 'spectrum because electrons bombard atoms and excite them.' ''The tube gives out red light 'that comes from neon atoms as they recover from that excitation.'' Because many electron transitions are possible, neon produces many other colours too.'' The hydrogen atom is much simpler than neon so there are fewer spectral lines.

This experiment was safety-tested in February 2006

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Beyond the visible spectrum

The Electromagnetic Spectrum
Light, Sound and Waves

Beyond the visible spectrum

Practical Activity for 14-16

Demonstration

This experiment can be used when introducing the electromagnetic spectrum. It shows that there is a form of radiation beyond the visible.

Apparatus and Materials

  • Prism, high-dispersion
  • Light source, compact
  • Positive lens, large
  • Screen, white
  • Infra-red detector, metered output
  • Infra-red detector, metered output

Health & Safety and Technical Notes

Read our standard health & safety guidance


For an infra-red detector, make a potential divider using a photo-diode in series with a 100kΩ resistor, and a 5 V supply across the pair. Connecting a digital voltmeter across the resistor will give the required metered output.

Compact light source: 100 W at 12 volts

The power supply should supply 8 amps.

Procedure

  1. Set up the compact light source. The lamp filament should be small enough that no slit is needed.
  2. Place the lens about 20 cm from the lamp. (If the lens is plano-convex, its plane face should be towards the lamp.) Move it to make an image of the filament on a white screen, 2 or 3 metres away.
  3. Place the prism just beyond the lens and move the screen round to catch the spectrum at the same distance from the lens as before but in the new direction. The spectrum will be pure enough for this demonstration if the prism is turned to minimum deviation. To make the spectrum longer, twist the screen to catch it obliquely.
  4. Move the detector across the spectrum near the screen and observe the output readings.

Teaching Notes

The radiation emitted by a hot filament has its maximum not in the visible spectrum, but beyond the red (in the 'infra-red' part of the spectrum). Light is just one (small) part of a family of radiations called the electromagnetic spectrum.

This experiment was safety-tested in February 2006

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Classroom management in semi-darkness

Interference
Light Sound and Waves

Classroom management in semi-darkness

Teaching Guidance for 14-16

There are some experiments which must be done in semi-darkness, for example, optics experiments and ripple tanks. You need to plan carefully for such lessons. Ensure that students are clear about what they need to do during such activities and they are not given unnecessary time. Keep an eye on what is going on in the class, and act quickly to dampen down any inappropriate behaviour before it gets out of hand.

Shadows on the ceiling will reveal movements that are not in your direct line of sight.

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The electromagnetic spectrum

The Electromagnetic Spectrum
Light Sound and Waves

The electromagnetic spectrum

Physics Narrative for 11-14

Radiation extends way beyond the visible spectrum, with both longer and shorter wavelengths. The longer the wavelength, the less energy a photon carries. The shorter the wavelength, the more energy it carries.

Infra-red radiation has a greater wavelength than visible light, and is easily measured with a coarse diffraction grating. Beyond the infra-red, radio waves continue the spectrum out through microwaves (short radio waves) to long radio waves with wavelengths measured in hundreds of metres.

Ultra-violet light has shorter wavelengths than visible light, measured by fine gratings operating in a vacuum to eliminate absorption by air. X-rays have far shorter wavelengths, usually less than 10-10m. A fine enough grating cannot be made, and so instead layers of atoms in a crystal are used.

These radiations are all part of the electromagnetic spectrum. You cannot see such a vast spectrum spread out across a screen. Only a tiny section is visible as light to which your eyes are sensitive. The methods of producing radiation in the various sections are different and so is their detection. Yet all the radiation, throughout the spectrum, travels with the same speed in a vacuum, and the radiation in each section can produce diffraction and interference effects. This shows that it consists of waves. These waves consist of varying electric and magnetic fields.

In diagrams showing the spectrum (see website)...

The Electronic Universe


...the wavelength scale used is one in which equal ratios of wavelengths are plotted rather than equal differences. For example, wavelengths 1 m and 100 m are spaced the same distance apart as wavelengths 10-2m and 1 m, and 10-6m and 10-4m. This is like the scale of octaves used in music, where a jump in the scale of one octave means a doubling of the frequency (and halving of the wavelength}. Such scales are called logarithmic.

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Learning from spectra

The Electromagnetic Spectrum
Light Sound and Waves

Learning from spectra

Teaching Guidance for 14-16

Introductory and intermediate level courses

Spectra are beautiful things to see. The full spectrum of white light blazing across a screen is a surprise and a delight to students, even though they have seen a ‘dilute’ version often enough in a rainbow. The full spectrum produced from white light is described as a continuous spectrum, as every colour is present across a range of colours.

Absorption spectra: Absorption spectra are produced when light from a source is partially absorbed by passing through a medium, for example, a beam of white light passing through a green bottle. At intermediate level, colour filters are used to help students to understand the physics of colour. These too are generally continuous spectra.

Line spectra: The ‘single line’ spectrum of a salted flame can come as a surprise: instead of a broad band of yellow, students see a very narrow band of pure yellow, an emission spectrum. Passing white light through sodium vapour produces an absorption spectrum that has a narrow black line at exactly the same place in the yellow. With able students, you might pose the question, ‘how is it that some sources produce a continuous spectrum but others produce a line spectrum?’.

Advanced level courses

In advanced level courses, you can move on and explain why spectral lines are produced.

Energy levels in atoms: Historically, line spectra were among the earliest phenomena to give hints of energy levels and quantum behaviour in atoms, but the hint was not pursued by physicists until other phenomena pointed towards quanta, early in the 20th century. Scientists may now think of line spectra as offering clear evidence for energy levels. In the late 19th century, however, the phenomenon was still a great puzzle.

In advanced level courses, students can follow the argument from frequency differences through the quantum idea to energy levels. Colleagues in chemistry want students to know that atoms have well-defined, discrete energy levels. Amazing stability goes with that: atoms and molecules are completely elastic in collisions, up to a certain limit, above which they can store or release energy in discrete jumps. The experiments and reasoning which led to that knowledge are not directly relevant to its use in chemistry, and so this teaching in chemistry generally rests on simple assertion.

Electron interactions with atoms: Physicists study the stability of energy levels in atoms by experiments in which they bombard atoms of vapour or gas with electrons of known energy. Up to a certain energy, the bombarding electrons bounce off the target atom elastically. They transfer no energy to the atom, beyond the tiny share which is characteristic of the momentum exchange in an elastic collision. Bombarding electrons do not change the energy of the target atom at all. But above a certain minimum threshold value, bombarding electrons make an inelastic collision, giving a sharply defined amount of their energy to the target atom, which is changed to a higher state or energy level. That is shown by the Franck-Hertz experiment (Wikiipedia has information on this experiment} in which, originally, electrons bombarded mercury atoms in warm mercury vapour.

Wikiipedia


In a school laboratory, a similar experiment can be done with electrons bombarding an inert gas such as helium, using a Teltron tube. After inelastic collisions, the atoms of the target gas soon return to their ground state, emitting light as a spectral line. That links up well with a full study of line spectra.

Photon interactions with atoms: When photons bombard atoms, again there are contrasting cases of elastic and inelastic collisions, plain scattering of light and the Compton effect; and the various forms of the Raman effect. These too reveal discrete energy levels in atoms. They also suggest that radiation, from visible light to X-rays, transfers its energy in quanta. A useful animation showing photon emission corresponding to transitions of electrons between atomic energy levels is shown here...

BIGS website


The atomic hydrogen spectrum can be shown with a grating. That spectrum, the Balmer series, has only four lines in the visible region, so students will not realize that the lines are part of a great series. Therefore, to supplement such measurements, you need to show a photograph of the Balmer series extending out into the ultra-violet. Those who like arithmetic puzzles might use the Balmer formula to see if their measurements fit.

Using spectral lines in astronomy: When astronomers look at spectra of distant stars and nebulae, they see spectral lines which obviously come from familiar elements studied in laboratories. These lines reveal the elements present in stars, interstellar space and galaxies.

In the spectra of remote galaxies, however, these lines are shifted towards the red. The red shift is greater for those galaxies which are further away (as judged by other evidence that seems trustworthy). A shift towards the red means a change to a longer wavelength, and also to a lower frequency. This suggests that galaxies are moving apart, and hence that space itself is expanding.

IOP DOMAINS Physics CPD programme

The IOP is proud to present our new Forces CPD Domain

This is the first of a collection of videos for you to view ahead of our live support sessions for teachers and coaches of physics.

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