Question

Describe an experiment to show that light travels in a straight line path.

Answer 1

Experiment:
Take three cardboard squares of equal size. Locate the centre of each pieces of cardboard by drawing the diagonals.
With the help of a nail, make a hole at the centre of each cardboard. Now fix the three card boards on plasticine or on stands so that they remain upright.
Arrange the three card boards A, B and C, one behind the other such that their centres are in the same horizontal line. You can pass a knitting needle through the holes to confirm if they are in a straight line.

Now place a burning candle in front of the board C and look through the pinhole in board $A$. The flame will be clearly visible. This shows that light travels in a straight line. Now, move board $B$ slightly and again look through the pinhole in board $A$ as shown in the figure.
You will not be able to see the flame. This shows that light does not travel in a zig-zag way.
By this experiment, we can conclude that light travels in a straight line. This property of light is called rectilinear propagation of light.