We will demonstrate experimentally that light travel in staight line.​

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.

This is a demonstration of how lights travels in a straght line

it is called kinetic energy