Light split into colours — answer key

1. UQ1 · Practice

Options as numbered on the pupil sheet: 1 — Violet; 2 — Red; 3 — Green.

Model answer. A prism refracts each colour by a different amount: red the least and violet the most, with the other colours in between in spectrum order. Ray A is bent least from the direction of the incoming beam, so it is red; ray C is bent most, so it is violet; ray B, between them, is green.

A full answer includes

• A, bent least: red.
• B: green.
• C, bent most: violet.

Also accept

• A red; B green; C violet

Watch for

• “A is violet, because it is at the top, like the top of the spectrum chart.” — Look at how much each ray is bent, not where it is on the page. The ray bent least is red.
• “All three rays are bent equally; the glass only colours them.” — The colours are already in the white light. The prism refracts each one by a different amount, which spreads them apart.

2. UQ2 · Practice

Model answer. White light is a mixture of all the colours. In glass each colour is refracted by a slightly different amount, so the prism spreads them apart: this is dispersion. The band of colours it makes is the visible spectrum, with red, refracted least, at one end and violet, refracted most, at the other.

A full answer includes

• (1) mixture
• (2) dispersion
• (3) spectrum
• (4) violet

Also accept

• mixture; dispersion; spectrum; violet

Watch for

• “The prism adds the colours to the light, so white light is not a mixture.” — The colours were in the white light all along: a second prism turned the other way gathers them back into white.
• “The colour refracted most is red.” — Red is refracted least. Violet, at the other end of the spectrum, is refracted most.

3. UQ3 · Practice

Options as numbered on the pupil sheet: 1 — Yellow; 2 — Violet; 3 — Orange; 4 — Blue; 5 — Indigo; 6 — Red; 7 — Green.

Model answer. Red is refracted least and violet most, and the other colours lie between them in the order of the rainbow: red, orange, yellow, green, blue, indigo, violet. “Richard Of York Gave Battle In Vain” gives the first letters in this order.

A full answer includes

• Red, orange, yellow, green, blue, indigo, violet.

Also accept

• red, orange, yellow, green, blue, indigo, violet

Watch for

• “Indigo comes after violet.” — Violet is the last colour, at the end refracted most; indigo is between blue and violet.
• “Yellow comes before orange.” — Orange lies between red and yellow, as in a rainbow.

4. UQ4 · Practice

Model answer. From red to violet the wavelength gets shorter and the frequency higher. Blue is near the violet end, so compared with blue, red light has the longer wavelength and the lower frequency. In air both travel at the same speed, so the longer wavelength must go with the lower frequency.

A full answer includes

• Red light has the longer wavelength and the lower frequency.

Also accept

• Red: longer wavelength, lower frequency

Watch for

• “Red has the longer wavelength and the higher frequency.” — At the same speed, a longer wavelength goes with a lower frequency: fewer crests pass each second.
• “Red and blue light have the same wavelength but travel at different speeds in air.” — In air all the colours travel at practically the same speed. They differ in wavelength and frequency.

5. UQ5 · Practice

Options as numbered on the pupil sheet: 1 — Blue; 2 — Violet; 3 — Red; 4 — Orange.

Model answer. Red has the longest wavelength of the visible spectrum, and the wavelength gets shorter towards violet. In spectrum order these four are red, orange, blue, violet, which is also their order of wavelength from longest to shortest (and of frequency from lowest to highest).

A full answer includes

• Red, orange, blue, violet.

Also accept

• red, orange, blue, violet

Watch for

• “Violet, blue, orange, red: violet is bent most, so it must have the longest wavelength.” — The colour bent most has the shortest wavelength. Red, bent least, has the longest.

6. UQ6 · Practice

Model answer. (a) White light is a mixture of all the colours of the spectrum. Glass slows each colour by a slightly different amount, so each is refracted by a different amount where the light enters the prism and again where it leaves: violet most and red least. The colours therefore spread apart into a band, the visible spectrum. This is dispersion. (b) The green light is refracted again by the second prism, but it stays green: no new colours appear. If glass coloured light, the second prism would colour it again. So the prism does not make the colours; it only separates the colours that were already in the white light.

A full answer includes

• (a) White light is a mixture of colours; the glass refracts each colour by a different amount (violet most, red least), so they spread apart: dispersion.
• (b) The green light is bent again but stays green; so the prism does not colour the light, it only separates colours that were already in it.

Also accept

• (a) mixture of colours, each refracted by a different amount (b) stays green; the colours were already in white light

Watch for

• “The prism splits the green light into more colours.” — Green from the spectrum is already one narrow band of colour; the second prism bends it but it stays green.
• “The prism spreads the colours because it reflects each one off a different face.” — The colours are separated by refraction, when the light enters and leaves the glass.

7. UQ7 · Practice

Model answer. Monochromatic light is light of a single frequency. A laser gives very nearly monochromatic light. A band of the spectrum, such as all the red light, is not monochromatic: it holds many slightly different frequencies.

A full answer includes

• Light of a single frequency.

Also accept

• light of a single frequency

Watch for

• “Light of one colour band, such as all the red light of the spectrum.” — The red band still holds many frequencies. Monochromatic means one frequency.
• “Very bright light, such as a laser beam.” — Brightness has nothing to do with it. Monochromatic means a single frequency, however bright or dim.

8. UQ8 · Practice

Model answer. (a) The card shows one green spot, bent away from the straight-on line by the prism, but not spread into a band of colours. (b) Laser light is monochromatic: it has a single frequency. A prism spreads light by refracting each frequency by a different amount, and with only one frequency there is nothing to spread apart, so the beam is refracted but stays green. (c) No: a sodium lamp’s light is also very nearly a single frequency, so a prism only bends it into one narrow yellow line, not a spectrum.

A full answer includes

• (a) One green spot, moved by the prism but not spread into colours.
• (b) Laser light is monochromatic: it has a single frequency, so there is nothing to separate; the beam is refracted but stays one colour.
• (c) No: its light is very nearly monochromatic, so it gives one yellow line, not a spectrum.

Also accept

• (a) one green spot, not spread (b) monochromatic: one frequency, nothing to separate (c) no; nearly monochromatic

Watch for

• “The laser beam is spread into a full spectrum, like sunlight.” — Sunlight is a mixture of very many frequencies. The laser gives one frequency, so it stays one colour.
• “The laser beam goes straight through, because it is too narrow to be refracted.” — Every beam is refracted at the faces of the prism. The laser beam is bent, but not spread.

9. UQ9 · Practice

Options as numbered on the pupil sheet: 1 — Gamma rays; 2 — Microwaves; 3 — The direction of increasing frequency; 4 — Ultraviolet.

Model answer. In order from radio waves, the seven regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. So P, between radio waves and infrared, is microwaves; Q, between visible light and X-rays, is ultraviolet; R, after X-rays, is gamma rays. The frequency rises from radio waves to gamma rays, so arrow S, pointing that way, shows the frequency increasing (the wavelength gets shorter that way).

A full answer includes

• P: microwaves.
• Q: ultraviolet.
• R: gamma rays.
• S: the direction in which the frequency increases.

Also accept

• P microwaves; Q ultraviolet; R gamma rays; S increasing frequency

Watch for

• “Q is infrared, because it is next to visible light.” — Infrared is on the other side of visible light, next to red. Q, between visible light and X-rays, is ultraviolet.
• “S shows the wavelength increasing.” — From radio waves towards gamma rays the wavelength gets shorter; it is the frequency that increases.

10. UQ10 · Practice

Model answer. (a) The rise is the final reading minus the starting reading: 20.6 °C − 18.0 °C = 2.6 °C. (b) The control rose by 18.2 °C − 18.0 °C = 0.2 °C, because the room warmed a little; taking that away, the radiation warmed the bulb by 2.6 °C − 0.2 °C = 2.4 °C. (c) The thermometer beyond the red warmed more than those in the green (0.9 °C) and the red (1.8 °C), yet no light could be seen there. Invisible radiation from the Sun, refracted less than red light, reaches that part of the card and carries energy into the bulb: this is infrared.

Checked automatically

• (a) Rise just beyond the red: 2.6 °C (within ±0.05 °C)
  • Common wrong answer 20.6 : That is the final reading. Subtract the starting reading, 18.0 °C.
  • Common wrong answer 2.4 : That already has the control’s rise taken away; that is part (b).
• (b) Rise caused by the radiation: 2.4 °C (within ±0.05 °C)
  • Common wrong answer 2.6 : Take away the control’s rise of 0.2 °C.
  • Common wrong answer 2.8 : Take the control’s rise away; do not add it.

A full answer includes

• (a) 20.6 − 18.0 = 2.6 °C.
• (b) 2.6 − 0.2 = 2.4 °C.
• (c) The biggest rise is beyond the red, where nothing is seen: invisible radiation, infrared, arrives there and heats the bulb.

Also accept

• (a) 2.6 °C (b) 2.4 °C (c) invisible radiation beyond the red heats the bulb: infrared

Watch for

• “The rise is 20.6 °C.” — That is the final reading. Take away the starting reading of 18.0 °C.
• “Nothing arrives beyond the red, because the card looks dark there.” — The bulb there warmed most. Your eyes cannot detect infrared, but a black bulb absorbs it.

11. UQ11 · Practice

Options as numbered on the pupil sheet: 1 — Microwaves; 2 — Gamma rays; 3 — Visible light; 4 — Radio waves; 5 — Ultraviolet; 6 — X-rays; 7 — Infrared.

Model answer. In order of increasing frequency the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. The wavelength goes the opposite way, so in order of wavelength, shortest first: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.

A full answer includes

• Gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.

Also accept

• gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves

Watch for

• “Radio waves first, because they come first in the spectrum.” — Radio waves have the longest wavelength. Shortest first starts with gamma rays.
• “Infrared comes between visible light and ultraviolet.” — Infrared is beyond the red end, with longer wavelengths than visible light; ultraviolet is beyond the violet end.

12. UQ12 · Practice

Model answer. (a) Beyond the violet end of the spectrum is ultraviolet radiation. The security ink absorbs it and glows with visible light. (b) Ultraviolet reaches the card there too, but your eyes cannot detect it, so that part of the card looks dark. (c) Ultraviolet is beyond violet in the spectrum, refracted even more, so it has a higher frequency and a shorter wavelength than violet light.

A full answer includes

• (a) Ultraviolet.
• (b) Eyes cannot detect ultraviolet, so the card reflects nothing we can see there.
• (c) Ultraviolet has the higher frequency (and the shorter wavelength).

Also accept

• (a) ultraviolet (b) eyes cannot detect it (c) ultraviolet

Watch for

• “Infrared made it glow.” — Infrared lies beyond the red end. Beyond the violet end is ultraviolet.
• “Violet light has the higher frequency, because it is visible.” — Being visible says nothing about frequency. Ultraviolet lies beyond violet, with a higher frequency.

13. UQ13 · Practice

Model answer. Infrared lies beyond the red end of the visible spectrum, between visible light and microwaves. It has a longer wavelength and a lower frequency than red light. It is not a colour: your eyes cannot detect it however bright it is, but a blackened thermometer bulb or skin absorbs it and warms up.

A full answer includes

• Infrared has a longer wavelength and a lower frequency than red light.

Also accept

• longer wavelength, lower frequency than red

Watch for

• “Infrared is red light that is too faint to see.” — Infrared is not a colour at all. It has a longer wavelength than red light, and eyes cannot detect it at any brightness.
• “Infrared has a shorter wavelength than red light, because it lies beyond it.” — It lies beyond the red end, on the side of longer wavelengths, towards microwaves.

14. UT1 · Apply

Model answer. (a) Spreading white light into its colours by refraction is dispersion. (b) Red is refracted least, so it lands nearest the straight-on direction of the beam; violet, at the other end, has the highest frequency. (c) The lamp also gives out infrared, which the prism refracts even less than red light, so it lands just beyond the red end. Eyes cannot detect it, but the sensor of a phone camera can detect some of it and shows it as a glow. (d) Beyond infrared, with even longer wavelengths and lower frequencies, come microwaves. (e) A sodium lamp gives light of very nearly a single frequency: it is monochromatic. The prism bends it but has no different frequencies to spread apart, so the screen shows one narrow yellow line instead of a spectrum.

A full answer includes

• (a) Dispersion.
• (b) Red is nearest the straight-on direction (refracted least); violet has the highest frequency.
• (c) Infrared: invisible radiation refracted less than red light, which the camera’s sensor can detect but the eye cannot.
• (d) Microwaves.
• (e) Supplement: one narrow yellow line, not a spectrum; the light is very nearly monochromatic, a single frequency, so the prism bends it but cannot spread it.

Also accept

• (a) dispersion (b) red nearest; violet highest frequency (c) infrared (d) microwaves (e) one yellow line; monochromatic

Watch for

• “The glow beyond the red is red light too faint for the eye.” — It is infrared: not a colour, with a longer wavelength than red light.
• “Ultraviolet is on the other side of infrared from visible light.” — Ultraviolet is beyond the violet end. Beyond infrared come microwaves.
• “The sodium lamp also gives a full spectrum, only dimmer.” — Its light is very nearly one frequency, so there is nothing to spread out.