question about coatings

How thick does an optical coating need to be to affect the refractive index of all the light passing through it? Can it be merely molecular thickness, or does it need more to get all of it?

Jess Tauber

jess tauber said:
How thick does an optical coating need to be to affect the refractive index of all the light passing through it? Can it be merely molecular thickness, or does it need more to get all of it?

Assuming you're talking about coatings on objectives, eyepieces, and the like, the purpose of them is not to affect the refractive index of the underlying glass (which they can't do, anyway), but to minimize reflection. In the simplest case, those coatings need to be about half a wavelength thick--about 275 nm for the middle of the visible light range. Compare that to the sizes of atoms, which are generally about 0.1 nm across. The molecules used in such coatings--such as magnesium fluoride (MgF2)--are generally on the smallish side.