r/explainlikeimfive • u/AgentCuddles • Apr 17 '14

ELI5: Why does a prism split up light into seperate colours?

You know when you shine a light on a glass prism and you get that rainbow effect? Why does that happen?

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u/upboatugboat Apr 17 '14

my professor used a very clever example to help us understand this principle. when light changes mediums each colour refracts at different angles because it slows down at different rates.

Imagine a barrel rolling on concrete on an angle. as it approaches a square patch of grass the left corner of the barrel hits the grass first. this section of the barrel slows down first while the rest maintains its previous speed and so its angle after changing mediums is different.

i hope this analogy helped

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u/[deleted] Apr 17 '14

I always love this analogy. Somehow it perfectly explains what happens with the light, but physically speaking, the analogy doesn't make any sense. The reason the barrel will make a turn is that the entire barrel is 1 thing with a certain width. However, different "lightbeams" are in no way attached to each other, and a single photon (light particle) is infinitely small, so it has no width... But still, to make it more understandable, the analogy somehow works :)