Why does a glass prism spread white light into a spectrum?

Glass slows each colour by a slightly different amount, so each colour bends at its own angle

Each colour is reflected off a different part of the glass

The glass adds colours that were not in the white light

How does the refractive index of glass compare for red and violet light?

It is higher for red than for violet

It is the same for every colour

It is higher for violet than for red

A Prism Splits White Light

Age 15 · Physics

Open to everyone until Thursday, October 29

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White light is a mix of every colour, and glass slows each colour by a slightly different amount, so each one bends at its own angle when it crosses a surface (Snell-Descartes law). Red bends the least and violet bends the most, so the colours fan out into a spectrum.

In the picture

Incoming white light: a mix of all the colours travelling together
Entry face: the light bends as it passes from air into glass, and each colour bends slightly differently
Red light: smallest refractive index, deviated the least
Violet light: largest refractive index, deviated the most

Examples

A rainbow forms when sunlight is refracted and dispersed inside raindrops
The bevelled edge of a mirror or a glass ornament throws small rainbows on a sunny wall

Key takeaways

The refractive index of glass depends on colour: it is higher for violet than for red
The spectrum always comes out in the same order: red, orange, yellow, green, blue, violet

Quiz: A Prism Splits White Light (Age 15)

1
When white light passes through a glass prism, which colour is deviated the most?
- Red
- Violet
- Green
2
Why does a glass prism spread white light into a spectrum?
- Glass slows each colour by a slightly different amount, so each colour bends at its own angle
- Each colour is reflected off a different part of the glass
- The glass adds colours that were not in the white light
3
How does the refractive index of glass compare for red and violet light?
- It is higher for red than for violet
- It is the same for every colour
- It is higher for violet than for red

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Physics, Age 15