What makes wet grass greener than dry grass? A scientific explanation of more than just refractive index would be much appreciated.
Actually, refractive index doesn’t explain it very much at all. There is surface reflection at the interface between grass and air that is due to the change in index of refraction (grass different from air). The index of refraction of water is closer to that of grass than air, so it helps the situation. Surface reflection is not colored (not green in the case of grass), but the same color as the illumination. Dry grass has a fairly diffuse surface that scatters that surface reflection in all directions so what you see is a combination of the green (subsurface reflection) plus the white (surface reflection). When the grass is wet, it becomes glossy since the water smoothes the surface. That means all the surface reflection goes off at one angle and becomes a glossy highlight. At other angles all you are left to see is the subsurface (green) reflection and thus the wet grass looks greener (more chromatic) than dry grass.
