Light travels in a straight line through an homogenous

medium. It can be proved by an experiment given below.

Arrange two pieces of cardboard, having pin holes at the

centre of each, vertically, one behind the other. They

should be arranged in such a way that the two holes are

exactly in the same level with each other. Now place a

candle behind and in level with the hole of the second

cardboard. It will be visible when viewed through the hole

of the first cardboard. Now displace one of the cardboards

slightly to its life or right candle would no longer be

visible. This shows that light travels in a straight line.