Find the refraction angle and critical angle when light crosses a boundary.
When light passes from one transparent medium into another, it bends. Snell's law relates the angles to the refractive indices: n₁sinθ₁ = n₂sinθ₂. Light slows down and bends toward the normal when entering a denser medium, and speeds up and bends away when entering a less dense one.
When light travels from a denser to a rarer medium, there is a critical angle beyond which it cannot escape and is instead totally internally reflected. The critical angle is arcsin(n₂/n₁), and exists only when n₁ is greater than n₂. This effect makes optical fibres and diamonds work. The calculator shows a dash when no critical angle exists.
It is the ratio of the speed of light in vacuum to its speed in the medium. A higher index means light travels more slowly and bends more.
Only when light moves from a denser to a less dense medium and the angle of incidence exceeds the critical angle.