In a homogenous transparent medium light travels in a straight line and this phenomenon is known as rectilinear propagation of light.
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, after which candle would no longer be visible. This shows that light travels in a straight line
