Light travels in straight lines which can be demonstrated by placing three cardboards A, B and Cand make a pinhole at their centres. Place a burning candle on one side of the cardboard A and arrange the cardboards in such a way that the three pinholes and the candle flame are in a straight line. The candle flame will be visible through the pinhole of the cardboard C. If any one of the cardboards is slightly displaced the flame will not be visible. From this it is clear that light travels in a straight line.