IGCSE Physics Dispersion of Light Questions and Worked Solutions (3.2.4) | 0625

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IGCSE Physics 0625 · Topic 3.2.4

IGCSE Physics Dispersion of Light Questions and Worked Solutions

50 topic-wise practice questions on Dispersion of Light (Unit 3: Waves), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions25 Core15 Extended10 Challenge112 marksTextbook §3.2 · pp. 136–159
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Dispersion of Light

White light is a mixture of colours. A glass prism separates them because each colour is refracted by a slightly different amount — this is dispersion. Violet is refracted most and red least.

Key formulas

c = f λspeed of light (3.0 × 10⁸ m/s), frequency, wavelengthExtended

Prerequisite knowledge

  • Refraction (3.2.2)
  • v = fλ (3.1)

Exam tips & common mistakes

  • Higher frequency means shorter wavelength
  • Frequency stays the same when light enters glass; speed and wavelength decrease
  • A rectangular block does not produce a visible spectrum because its faces are parallel

Key facts from the syllabus (3.2.4)

  • Dispersion: white light split into colours by a prism
  • Seven colours in order of increasing wavelength: violet, indigo, blue, green, yellow, orange, red
  • In order of increasing frequency: red, orange, yellow, green, blue, indigo, violet
  • Violet is refracted most; red least
  • Extended light of a single frequency is monochromatic (e.g. laser light)

Textbook: §3.2 Light, pp. 136–159

1
Core[2 marks]

Describe what happens when white light passes through a glass prism.

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Method: Dispersion.

It is split into a spectrum of colours (dispersion) because each colour is refracted by a different amount
2
Core[1 mark]

Name the process in which white light splits into colours.

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Method: Dispersion.

Dispersion
3
Core[2 marks]

List the seven colours of the visible spectrum in order of increasing wavelength.

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Method: Violet shortest, red longest.

Violet, indigo, blue, green, yellow, orange, red
4
Core[2 marks]

List the seven colours of the visible spectrum in order of increasing frequency.

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Method: Red lowest frequency.

Red, orange, yellow, green, blue, indigo, violet
5
Core[1 mark]

State which colour of light is refracted most by a prism.

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Method: Violet bends most.

Violet
6
Core[1 mark]

State which colour of light is refracted least by a prism.

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Method: Red bends least.

Red
7
Core[1 mark]

State which colour of visible light has the longest wavelength.

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Method: Red.

Red
8
Core[1 mark]

State which colour of visible light has the highest frequency.

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Method: Violet.

Violet
9
Core[2 marks]

Explain why different colours are separated by a prism.

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Method: Different speeds in glass.

Each colour travels at a slightly different speed in glass, so each is refracted by a different amount
10
Core[2 marks]

Explain how a rainbow forms.

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Method: Raindrops disperse sunlight.

Sunlight is refracted and dispersed by raindrops (and reflected inside them), separating the colours
11
Core[1 mark]

State the colour between yellow and blue in the visible spectrum.

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Method: Green.

Green
12
Core[2 marks]

A spectrum is projected onto a screen. State which colour is nearest the base of the prism (most deviated).

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Method: Violet deviated most.

Violet
13
Core[2 marks]

Explain why light of one colour passing through a prism is not split further.

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Method: Single frequency.

It contains only one frequency, so all of it is refracted by the same amount
14
Core[2 marks]

State which colour has the shortest wavelength in visible light.

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Method: Violet.

Violet
15
Core[2 marks]

State which colour has the lowest frequency in visible light.

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Method: Red.

Red
16
Core[2 marks]

Explain why white light is described as a mixture of colours.

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Method: Prism splits it.

A prism splits it into a spectrum of colours
17
Core[2 marks]

Describe how to show that the colours of a spectrum can recombine to form white light.

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Method: Second prism.

Place a second, inverted prism in the path of the spectrum; the colours recombine into white light
18
Core[2 marks]

State which colour is between orange and green in the spectrum.

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Method: Yellow.

Yellow
19
Core[2 marks]

State the colour that is deviated more by a prism: blue or orange.

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Method: Higher frequency bends more.

Blue
20
Core[2 marks]

State the colour order seen in a rainbow from outside to inside.

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Method: Red outside.

Red, orange, yellow, green, blue, indigo, violet
21
Core[2 marks]

Explain why a prism does not disperse light when the light enters and leaves along the normals.

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Method: No refraction.

There is no change of direction at either face, so the colours are not separated
22
Core[2 marks]

State which colour of light travels slowest in glass.

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Method: Violet.

Violet
23
Core[2 marks]

Explain why white light entering a rectangular block is not spread into a visible spectrum when it leaves.

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Method: Parallel faces.

The opposite faces are parallel, so the colours emerge parallel and overlap again
24
Core[1 mark]

State the name for the band of colours produced by a prism.

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Method: Spectrum.

A (visible) spectrum
25
Core[2 marks]

State the colours at each end of the visible spectrum.

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Method: Red and violet.

Red and violet
26
Extended[1 mark]

Define monochromatic light.

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Method: Single frequency.

Light of a single frequency
27
Extended[1 mark]

Give an example of a source of nearly monochromatic light.

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Method: Laser.

A laser
28
Extended[2 marks]

Explain why a laser beam passing through a prism produces a single spot rather than a spectrum.

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Method: Monochromatic.

Laser light is monochromatic (one frequency), so it is all refracted by the same amount
29
Extended[2 marks]

State the relationship between the frequency and wavelength of visible light in a vacuum.

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Method: v = fλ with constant speed.

Inversely proportional (higher frequency, shorter wavelength) since speed is constant
30
Extended[3 marks]

Red light has a wavelength of 7.0 × 10⁻⁷ m. Calculate its frequency (c = 3.0 × 10⁸ m/s).

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Method: f = c ÷ λ.

f = 3.0×10⁸ ÷ 7.0×10⁻⁷ = 4.3 × 10¹⁴ Hz
31
Extended[3 marks]

Violet light has a frequency of 7.5 × 10¹⁴ Hz. Calculate its wavelength (c = 3.0 × 10⁸ m/s).

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Method: λ = c ÷ f.

λ = 3.0×10⁸ ÷ 7.5×10¹⁴ = 4.0 × 10⁻⁷ m
32
Extended[2 marks]

Explain why white light is not monochromatic.

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Method: Mix of frequencies.

It is a mixture of many frequencies (all the colours of the spectrum)
33
Extended[2 marks]

Explain why monochromatic light is used for some experiments.

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Method: Single wavelength.

It has a single frequency/wavelength, so it is not dispersed and gives sharp results
34
Extended[3 marks]

Calculate the wavelength of green light of frequency 6.0 × 10¹⁴ Hz (c = 3.0 × 10⁸ m/s).

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Method: λ = c/f.

λ = 5.0 × 10⁻⁷ m
35
Extended[3 marks]

Light of wavelength 6.0 × 10⁻⁷ m travels in air. Calculate its frequency.

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Method: f = c/λ.

f = 5.0 × 10¹⁴ Hz
36
Extended[2 marks]

State whether frequency or wavelength stays constant when light enters glass.

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Method: Frequency.

Frequency stays constant; wavelength decreases
37
Extended[3 marks]

Red light (λ = 7.0 × 10⁻⁷ m in air) enters glass with refractive index 1.5. Calculate its wavelength in the glass.

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Method: λ falls by factor n.

λ = 7.0×10⁻⁷ ÷ 1.5 = 4.7 × 10⁻⁷ m
38
Extended[2 marks]

A laser produces light of a single wavelength. State the word that describes this.

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Method: Monochromatic.

Monochromatic
39
Extended[2 marks]

Explain why a rainbow can be seen only when the Sun is behind you.

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Method: Geometry of dispersion/reflection.

Sunlight must enter raindrops in front of you and be dispersed and reflected back towards you
40
Extended[2 marks]

Explain why the refractive index of glass is slightly different for each colour.

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Method: Speed differs.

Each colour travels at a slightly different speed in glass

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Explain why white light splits into a spectrum when passing through a prism but not through a rectangular glass block.

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Method: Non-parallel faces.

Each colour refracts by a different amount; in a prism the second refraction spreads the colours further; in a block the parallel faces cancel the deviation so colours emerge parallel and overlap
42
ChallengeCore[4 marks]

List the colours of the spectrum in order of increasing wavelength and state which is deviated most by a prism, explaining why.

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Method: Violet slowest in glass.

Violet, indigo, blue, green, yellow, orange, red; violet is deviated most because it travels slowest in glass (highest refractive index)
43
ChallengeCore[3 marks]

Describe how to show that white light can be recombined from a spectrum.

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Method: Second prism.

Pass the spectrum through a second identical prism turned the other way; white light emerges
44
ChallengeCore[3 marks]

Explain how raindrops produce a rainbow.

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Method: Refraction, dispersion, reflection.

Sunlight refracts and disperses on entering a drop, reflects inside it, and refracts again on leaving, separating the colours
45
ChallengeExtended[4 marks]

Red light (4.3 × 10¹⁴ Hz) and violet light (7.5 × 10¹⁴ Hz) travel in a vacuum. Calculate both wavelengths.

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Method: λ = c/f.

Red: 7.0 × 10⁻⁷ m; violet: 4.0 × 10⁻⁷ m
46
ChallengeExtended[4 marks]

Glass has refractive index 1.51 for red light and 1.53 for violet light. Calculate the angles of refraction for each at an angle of incidence of 50°.

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Method: Snell.

Red: 30.5°; violet: 30.0°
47
ChallengeExtended[4 marks]

Light of wavelength 6.0 × 10⁻⁷ m in air enters glass of refractive index 1.5. Calculate its frequency and its wavelength in the glass.

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Method: f unchanged.

f = 5.0 × 10¹⁴ Hz; λ = 4.0 × 10⁻⁷ m
48
ChallengeExtended[3 marks]

Explain why a laser beam passing through a prism stays as a single narrow beam.

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Method: Monochromatic.

It is monochromatic — one frequency — so all of it is refracted by the same angle
49
ChallengeExtended[4 marks]

Explain why the speed, wavelength and frequency of green light change or stay the same as it passes from air into water.

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Method: f fixed by source.

Speed decreases, wavelength decreases, frequency stays the same
50
ChallengeExtended[3 marks]

Explain why the colours of a spectrum have no sharp boundaries between them.

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Method: Continuous range.

White light contains a continuous range of frequencies, so the colours merge gradually
All IGCSE Physics topics← 3.2.3 Thin Lenses3.3 Electromagnetic Spectrum →

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