Quote Originally Posted by Photo Engineer View Post
There is always an interface whether the layers are singly coated or coated in multiples at one time. There are tiny refractive index changes due to the chemicals present in the layers due to the fact that layers differ in content. The difference within the layer itself is substantial.
If that difference is so high, what about the explanation, that gelatin hardening might play an important role?

Think of the lens analogy. You have to coat lenses even though you may use the same glass in every element. It is the interface that counts even if there is no air there.

Far from being particularly knowledgeable about optics I assume that anti-reflection coatings only make sense when put at the air-glass interface.