• The short answer is that atoms, due to the energy state of their electrons (which valence shell they're in and how many electrons) will reflect and absorb photons of different states--light of different colors. That's why different substances are different colors. Now, if we take the light coming from something, it's color is going to be the composite of the light wave frequencies it's reflecting. By putting the light into a prism, to break the (usually) whitish light into its composite color (light frequencies), it tells you what the light's made of. Now since we've studied the properties of so many substances, including what light they reflect. So, we can match what we know to what we see in the prismed white light, and there you have it: a list of what the light reflected from. It's a little different process depending on if it's reflected light (as in optical minerology) or light produced through a chemical or atomic process (as with star light) because the properties vary, but it's all the same process. Hope that helped!

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