Colours bending differently
What Does a Prism Do?
A prism splits white light because each colour bends by a different amount.
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Step 1 of 5
White light is a mixture.
It looks like one thing but is every colour together. A prism does not colour the light — it separates what was already there.
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Step 2 of 5
Light bends when it changes material.
Entering glass it slows down and changes direction, and leaving it speeds up and bends again. That bending is refraction.
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Step 3 of 5
Each colour bends differently.
Short violet waves are slowed most and bend most; long red waves bend least. That difference is the entire mechanism.
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Step 4 of 5
A triangle shape doubles the effect.
The two faces are not parallel, so the second bend adds to the first instead of cancelling it. In a flat pane the bends cancel and nothing separates.
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Step 5 of 5
Raindrops do the same thing.
A rainbow is millions of raindrops each acting as a tiny prism, splitting sunlight and reflecting it back to you.
Newton proved the colours were in the light, not the glass, by recombining them with a second prism to make white again.
The short version
A prism separates white light because each colour bends by a different amount, and its angled faces add the bends together.
Try it yourself
Put a glass of water on a sunny windowsill with paper behind it. A rainbow appears on the paper.
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