Answer:
The light always travels in a straight line. This can be shown by performing a simple activity. For this, take three cardboards of equal size are required. Now, a hole is made at the center of each cardboard at the same level. The three cardboards are placed on a flat table and aligned linearly as shown in the figure. 
A candle is lighted at one end of the table with its flame at the level of the holes and is seen at the other end. It is observed that the light of the candle is seen effortlessly. Now, the middle one of the three cardboards is displaced from its position. Now, we are unable to see the light. The reason behind this is that the light travelling in the straight line is obstructed due the misalignment of the cardboards. This proves that light always travels in a straight line. This is called as Rectilinear propagation of light.
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