In terms of the wavelength of light in magnesium fluoride, what is the minimum thickness of magnesium fluoride coating that must be applied to a glass lens to make it non-reflecting for that wavelength? (The index of refraction of magnesium fluoride is intermediate to that of glass and air.)

Huygen's principle: Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets - that is, tangent to all of them.