# Is light getting another color when travelling through a prism?

When white light travels through a prism the speed of this light is slowed down. Now there is a relation between speed of light, frequency and wavelength.

So when a red light of 700nm enters a prism it will slow down at 0,9c. So wavelength = speed / frequency. Now you can theoretically divide the changes. So 600nm = 0,9 / 0,0015 or you can have 800nm = 0,9 / 0,0011.

So the color in the prism can turns to 600nm or 800nm. Probably after leaving the prism the color gets back its original color.

But is it the case while travelling through the prism the color changes? Probably very tiny?

• What, exactly, do you mean by "color" inside the prism? "Color" describes our perception of light falling into our eyes, but light inside the prism can't fall into our eyes. – ACuriousMind Apr 15 '16 at 18:52
• wiht color I mean the wavelength – Marijn Apr 15 '16 at 18:53
• Of course the wavelength changes! The frequency stays constant, and the speed changes, so the wavelength also has to change. I'm not sure I understand your question. – ACuriousMind Apr 15 '16 at 18:59
• Perhaps that is the answer but I couldn't find it anywhere – Marijn Apr 15 '16 at 19:00
• But is it possibel that it is the other way, that the frequency changes and the wavelength stays the same – Marijn Apr 15 '16 at 19:01

When light enters glass (or another transparent material), its frequency stays the same and its wavelength changes.

In a comment, you say that you are using "color" to mean "wavelength". Well, I think you are using the word "color" incorrectly. According to a normal definition of "color", the color of light does not change when it enters glass. But the wavelength does.

ADDENDUM: Color has to do with how light appears to you when you see it. You can't see light while it's in the prism, you only see it after it exits the prism and enters your eyeball. Yes, the wavelength changes when the light enters the prism, but it changes right back when it exits.

• I agree. I also think that a useful addition to this answer would be a link to (or statement of) a normal definition of "color" – Duncan Harris Apr 15 '16 at 19:23
• But why is the frequency the same and the wavelength changed what is the cause of it that it is not the opposite? And why is the color not changed as you can see the light travelling through the prism and color wavelength depending is – Marijn Apr 15 '16 at 19:23
• @Marijn Because the matching happens at the inter-medium boundary, and there the frequencies need to match but the boundary cannot probe what the wavelength is inside the medium as that is a bulk effect. (cf. e.g. this perspective.) – Emilio Pisanty Apr 15 '16 at 19:24
• @Marijn Frequency has to stay the same when waves go from one medium to the next because each wave that leaves the first medium produces exactly one wave in the new medium. – M. Enns Apr 15 '16 at 20:48

This is a phenomenon called dispersion of light.

White light is a mixture of all colours which get separated when they pass through a prism. The refractive index or simply for lay usage the angle at which light bends when it enters from one medium to another depends on the wavelength of light. So the colours are not newly created, but they are just separated into a spectrum of light because of their difference in wavelengths, red with the highest and violet with the lowest wavelengths in the visible spectrum. So these two colours are at the extremities of the dispersion and the other colours being in the middle arranged in the order of VIBGYOR based on their respective wavelengths.

The same thing happens in the formation of a rainbow...one of nature's spectacular shows. I hope you got it.

• I got this, but what is happening inside the prism with the wavelength and frequentcy? – Marijn Apr 15 '16 at 20:38
• speed of light depends on the refractive index of the medium as well as the wavelength of light.The dispersion happens only because of one reason differences in the speeds of respective wavelengths.Speed of light is around 3×10^8 m/s in vacuum but in any medium it always decreases because it takes some time for the atoms in the intervening medium to absorb and emit photons.This decrease in speed is reflected as decrease in wavelength and with some math you could get that the other factor...frequency remains same.So in a nutshell the wavelength decreases in the prism but the frequency is same. – White Knight Apr 15 '16 at 21:28

## protected by Qmechanic♦Apr 16 '16 at 0:23

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