Refraction and change of speed

When light passes form one medium to another of different refractive index its speed changes. It moves more slowly in a material of higher refractive index than it does in a material of low refractive index.

The light moves slower in the material of higher refractive index (medium 1).
n₁sinq₁ = n₂sinq₂ and n₁/n₂ = c₂/c₁
therefore: c₂sinq₁ = c₁sinq₂

Refractive index = velocity in free space (usually taken as air)/velocity in the material

The ratio of the speeds in the two materials is the inverse ratio of the refractive indices of the two materials