Light reflected from the surface of a transparent medium is found to be completely plane polarised at an angle of incidence of 60°. The refractive index of the medium is:

Two coherent slits S₁ and S₂ act as secondary sources; at each point on the screen, the path difference between light from S₁ and S₂ determines whether the waves arrive in phase (bright fringe) or out of phase (dark fringe), producing the characteristic alternating fringe pattern.