Refraction through a Rectangular Glass Slab

• The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence all lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction. (This is true for angle 0 < i < 90°)
  If i is the angle of incidence and r is the angle of refraction, then,
  \[\frac {\text{sin i}}{\text{sin r}}\] = constant

This constant value is called the refractive index of the second medium with respect to the first.