You can see this in "real life" if you take a glass filled with water and look into it. You can't see out of the glass, but if you press your finger to the side

You can see this in "real life" if you take a glass filled with water and look into it. You can't see out of the glass, but if you press your finger to the side you'll see your fingerprint appear. You've changed the refractive index and change the reflection/refraction properties.