Question

Why does abstaine change colour once mised with water?

Answer 1

As others have noted, adding a substance to make the water change color does not by itself change the refractive index of the water to a significant degree. However, there’s a twist. The absorption spectrum of a substance (that is, to what extent light of different wavelengths, or colors if you prefer, are absorbed) and the refractive index are really two sides of the same coin. They are the real and imaginary part of the complex index of refraction, and are related via the so called Kramers-Kronig relations. So if you have a solution which absorbs light at all frequencies except in one narrow band (giving rise to a color) you can bet that the refractive index close to the frequency band will be affected as well!



The diffraction index of the water (or any "transparent" medium) does not depend from the colour of the water but only from the wavelength and the so called wavelength dispersion.

However, the colour of the water influence what colours will pass through it.

Let's assume that you have a white light beam falling on water surface and the water is coloured yellow. Then the all colours will diffract following the Snell's Law of diffraction with the appropriate diffraction index (dispersion). However, the yellow colour light beam will pass through the yellow coloured water while all other colours will be absorbed (absorbed from 0% to 100%, depending from the water layer thickness and the yellow colour "saturation").

This is the reason when you dive into the sea, the surrounding water and objects take a "bluish" colour; all other wavelengths, except the blue, are quickly absorbed from the water column above you (of course, the blue colour is also slowly absorbed; eventually in the open sea all sun light is absorbed at depths beyond 300-500m).