Light passing through a composite block

Consider light passing through the composite block composed of two parallel-sided slabs of transparent material of refractive indices n₁ and n₂ as shown in the diagram.

Considering the refractions at the three boundaries, we have:

n₁ = sini/sini_{1 1}n₂ = sini₁/sini_{2 2}n = sini₂/sini = ₁/n₂

and this gives us the important relationship that:






This can be extended to cover the passage of light through a multiple block. Notice also that if the block is parallel- sided (as shown in the diagram) then the light emerges travelling in the same direction as its original path but displaced sideways (see below). You should expect this result from the principle of reversibility of light.