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Crystal leaving patterns of the electromagnetic spectrum?

  1. Jul 26, 2016 #1
    I was bored today so I took this diamond shaped crystal made of glas and simply put in infront of the light emitting from the sun. What I noticed was a lot of lines on the wall that were strangely colored, just like the electromagnetic spectrum. One end was blue, then progressively went to red, inbetween was green/yellow.

    Can someone explain how this happens? The photons from the sun entered the crystal in it's pointy area rather than the top of it.

    Bonus question: When I stared into the crystals radiation emitting from it, I noticed that the color keept on shifting all the time. First it was blue, then yellow, then red and green and so on. Why?
  2. jcsd
  3. Jul 26, 2016 #2


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    Light travels slower in glass (or any medium, which invariable has an index of refraction > 1) than it does a vacuum or in air. What's more the speed is dependent upon the frequency. Blue light travels slower than red light, in the medium. [Edit: All frequencies travel at the same speed in a vacuum. But not necessarily so in the medium.] That means, in the right configuration, that white light can get split into its constituent colors.

    Last edited: Jul 26, 2016
  4. Jul 26, 2016 #3


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    To expand on collinsmark answer, when the light from the Sun enters the crystal (or any boundary between two mediums that differ in their refractive index) the light changes direction slightly based on the difference between its speed in one medium and its speed in the other. Since the speed of light in a medium generally decreases with increasing frequency (a concept known as dispersion), blue light tends to slow down more than green or red light when entering something like glass or crystal. Since air has a refractive index very close to a vacuum, the difference in speed between air and crystal or glass is greater for blue light, and hence its direction changes more than the other colors lower in frequency.

    So when the light exits the crystal the different colors are no longer traveling in the same direction, leading to a separation of colors that you can see with your eye.
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