Practice questions · Dispersion

Red bends least. From the first face.

Six original Cambridge-style questions. They cover the cause of dispersion, the order of the spectrum, and the two drawing and reasoning traps: late dispersion and red bending the most.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Three things to keep straight

Different colours, different bending, from the first surface.

Cause. Each colour has a different refractive index in glass, so each refracts by a different amount.
Order. Red bends least (longest wavelength), violet bends most (shortest wavelength).
Where. The colours start to separate at the first air-to-glass boundary, not the second.

[2 marks]

Explain what is meant by dispersion of light, and state what it shows about white light.

Dispersion is the splitting of white light into its separate colours (a spectrum) when it refracts through a prism. ✓
It shows that white light is a mixture of many colours. ✓

[2 marks]

A prism disperses white light into a spectrum.

(a) List the colours of the spectrum in order. [1] (b) State which colour is deviated the most and which the least. [1]

(a) Red, orange, yellow, green, blue, indigo, violet. ✓

(b) Violet is deviated the most, red the least. ✓

Analysis

[3 marks]

Explain, in terms of speed and refraction, why red light and violet light are deviated by different amounts as they pass through a prism.

Different colours travel at slightly different speeds in glass, so glass has a slightly different refractive index for each. ✓
Violet is slowed the most, so it refracts and deviates the most. ✓
Red is slowed the least, so it refracts and deviates the least. ✓

Analysis

[2 marks]

A student draws white light passing straight and undivided through the first face of a prism, with the colours only appearing as it leaves the second face. Explain why this diagram is incorrect.

The colours begin to separate at the first air-to-glass boundary, because each colour refracts by a different amount as it enters the glass. ✓
So the light is already dispersing inside the prism. The diagram should show the colours fanning apart from the first surface, separating further at the second. ✓

Dispersion does not wait for the second face. It begins the instant the light enters.

Analysis

[2 marks]

A student claims that red light must bend the most through a prism because it has the highest energy. Explain what is wrong with this statement.

Red light has the longest wavelength and is slowed the least by the glass, so it actually bends the least. ✓
It is violet, with the shortest wavelength, that is slowed the most and bends the most. ✓

Longest wavelength, least bending. The "energy" reasoning leads to the wrong answer here.

Analysis

[2 marks]

White light from the Sun passes through raindrops and forms a rainbow, which is dispersion in action.

(a) State the property of light that determines how much each colour is deviated. [1] (b) A prism produces a spectrum with red at the top. State where violet appears, and why. [1]

(a) Its wavelength (which sets how much the glass slows and refracts it). ✓

(b) Violet appears at the bottom, because it is deviated the most while red is deviated the least. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.