Mathematically prove that when a light ray passes through a rectangular glass slab, the incident ray is parallel to the emergent ray. Illustrate with a diagram.

In triangle GBC,

r1+90-r2=90

so r1=r2

as ?=sin i/sin r1 =sin r2/sin e    as r1=r2

so i=e

so for, CF and DE, and intercector CD

angle BCF=angle CDE=corresponding angle

so CF parallel to DE.