# B Colour Change problem

1. Jul 2, 2017

### Zaya Bell

Whenerver a monochromatic light, say red changes medium, it reduces in speed which in turn reduces wavelength. But then if wavelength is reduced should not the colour change? Or dies it?

2. Jul 2, 2017

### Staff: Mentor

What exactly do you call "color"? What human eyes or cameras record? That fixes the medium to eyes/cameras, and whatever the light crosses before does not matter for the wavelength and frequency received there.

Typically frequency is a better measure for "colors" because it is medium-independent.

3. Jul 2, 2017

### Merlin3189

Just some odd thoughts provoked by these posts!

So perhaps one could say the colour had changed, even though there is no way of seeing it, but it always has to change back, to the wavelength/colour determined by its frequency, when it reaches the retina or camera sensor or whatever?

But what struck me, that I'd never thought of before, is, does it matter which way light passes through a diffraction grating? If you had a transmission grating of say glass with lines ruled/etched/engraved or whatever on one face, then the wavelength of light striking the grating would be different if it was in the air or in the glass.
I've never noticed any difference when you reverse the plate, so I wondered a bit more. It is the medium after the grating which is important in creating the path difference, not the medium before. Whichever way round the plate is, the light goes either none or a very short path through the glass and the rest of the distance to the screen/detector in air. So the glass would make very little difference to the path length.
I wonder if anyone has tried making a glass block say 1m long with a grating engraved on one end? I suppose not, since there would be no point, as we all know what would happen. But one could experiment with a diffraction grating under water say and compare it with the results in air. One could see that red light in water landed where blue green did in air, but it would still look red by the time it got into your eye.

And adding to mfb's point, is it not frequency that determines photon energy? Then which retinal pigment is affected by light depends on photon energy to interact with electrons of specific energy in the pigment molecule? So it really is frequency which determines colour, not wavelength.

4. Jul 3, 2017

### tech99

I think that the cone cells of the eye will detect wavelength rather than frequency - they are little antennas. The red light changes wavelength but not frequency when it passes through different media, but when it enters the eye, it travels through the Vitreous Humour in order to reach the Retina, and hence its velocity is standardised according the the permittivity of that medium.