Consider two parallel slabs having refractive index andare kept one above the other. The medium above and below the compound slab is the same.

If we consider refraction of light at the interface of ‘a’ and ‘b’, then

Now consider refraction of light at the interface of ‘b’ and ‘c’, then

Considering refraction of light at the interface of c and a, we get

Multiplying equations (1), (2) and (3), we get

or or