Reflection and refraction with equal refractive indices?

[Hypatio]

The Fresnel equations indicate that radiation will be 100% transmitted if two mediums have the same refractive indices. If that is true, then whey is there so much reflection off of, for instance, cracks in glass? Is this because there is a microscopic vacancy where the index of refraction becomes that of a vacuum or air, which is ~1?

Similarly, if there are two crystals with equal properties in near-perfect contact, will the boundary between the two be invisible?

 

[sophiecentaur]

If you are talking about an ideal situation then there would be no discontinuity - hence no reflection.
If you are talking about a practical situation then you would need to specify the geometry of the interface (spacing / angle etc) but you would expect some perturbation of the ray at the interface.

 

[DrDu]

I think it is the filling of the cracks with gas or vacuum of other index of refraction. You can demonstrate this by bringing a crystalline powder (sugar, salt or the like) in a glas. It will appear white and not transparent. However, if you fill in a liquid with the same index of refraction as the powder, the powder will appear transparent. In fact, this is used to determine the index of refraction in mineralogy.