Multiple panes of glass will do the trick," said another. Another user joked, "When earth has astigmatism." The refraction ...

A ray diagram showing refraction at the boundary between air and glass Refraction can cause optical illusions as the light waves appear to come from a different position to their actual source.

In all ray diagrams, all angles of incidence and refraction are measured between the ray and the normal. Watch this video for a practical demonstration of measuring the refractive index of glass.