# Colour of light in glass

1. Jan 11, 2012

### unnddd

We know that the wavelength of visible light changes when it travels between two media with different refractive indices. Here's my question: does light change colour when it moves from air into glass?

I had quite an interesting but inconclusive discussion about this with my Physics teacher today, with the main point of difficulty being the fact that colour is simply what humans perceive it to be. When you observe light being refracted, you see it after it's left the glass and so there's no way you can directly observe its colour when it's in the glass. You'd have the same problem with any measuring instrument.

I don't even know whether I'm asking the right question. Perhaps a better question would be: what is colour? I'd be grateful if somebody could help me obtain a clearer picture of refracted light, because I'm a little bit confused about this particular aspect of colour.

Thanks

2. Jan 11, 2012

### rollcast

The light does not change colour per se. Its just the various wavelengths that make up the light get spread out which in turn allows you to identify individual colours.

3. Jan 11, 2012

### unnddd

I'm not referring to how some wavelengths are refracted more; rather, I'm talking about the behaviour of a specific wavelength of light when it enters glass, and whether it changes 'colour', as we know it.

4. Jan 11, 2012

### rollcast

[STRIKE]The colour does change as the colour depends on wavelength (λ) and frequency.[/STRIKE]
The colour doesn't change as the colour depends on wavelength (λ) and frequency.

And these are linked via the equation

$\lambda=\frac{c}{f}$

Where λ = wavelength
c = speed of light (dependant on material)
f = frequency

<< Post edited by Mods at OP request >>

Last edited by a moderator: Jan 13, 2012
5. Jan 11, 2012

### Dr Lots-o'watts

The only wavelengths that matter are those within the vitreous humor of the eye.

6. Jan 11, 2012

### unnddd

Does the light have a colour if it isn't being observed?

This is the difficulty we faced when trying to establish whether the light would change colour. Given that in this thread there are differing opinions, I'm glad to know my teacher and I aren't alone in being a little bit unsure.

7. Jan 11, 2012

### Drakkith

Staff Emeritus
To clarify, light does not posess a "color", it only has wavelength and frequency and other properties such as spin, polarization, etc. Color is what we see after processing the light in our eyes and brains.

Light of a specific wavelength will leave a transparent material and still have the same wavelength. So no, it would not "change color". Shining a red laser through lots of glass would alter the path the light travels, but not its wavelength/frequency.

8. Jan 11, 2012

### unnddd

Thanks for the clarification; that's what we were thinking.

Just to make clear what I meant, I wasn't referring to the colour of the light after leaving the glass as that's obviously trivial.

9. Jan 11, 2012

### Dr Lots-o'watts

It's not an opinion! Colour is determined by whichever wavelengths were in the vitreous humor! It's an extra medium after air and glass and what not. No ambiguity!

10. Jan 11, 2012

### Drakkith

Staff Emeritus
I don't think the vitreous humor matters. Once the light reaches your cone cells they have to get through the pigments that cover them. These pigments determines which cells detect red, blue, or green light. So no matter what material was in front of these cells the light would still be the same once it enters them.

11. Jan 11, 2012

### clem

It5 is actually the frequency that matters as the light hits the retina, so there is no change in the observed color. This can easily be observed as light of a pure color goes through a large prism. The description of color in terms of wave length is just used as a convenient measure, but it is the wave length in vacuum that relates to color.

12. Jan 13, 2012

### Claude Bile

From a physical chemistry point of view, it is the frequency of the photon that initiates the necessary chemical reactions that allows the cones in our eyes to detect light. Since frequency is invariant, the light path taken by the photon does not affect what we perceive.

Claude.