Give an experiment to demonstrate that light travels in a straight 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.
- By this experiment, we can conclude that light travels in a straight line. This property of light is called rectilinear propagation of light.
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