Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Propagation of light through a rectangular hole


For large size holes, the light propagation is governed by geometrical optics (rectilinear propagation). However, you can create situations in which, at the first glance, some surprises occur. This experiment is one such example.


A card board fixed vertically with an arrangement to open a rectangular hole, an electric bulb holder, a filament bulb and a CFL, a vertical white screen.


  1. Arrange the bulb holder, the cardboard and the white screen in line as shown in the figure. Keep the distance between the bulb holder and the cardboard about 30 cm and, between the cardboard and the screen also about the same. Make rectangular hole thinnest. To do this keep the given hinges closed.
  2. Put a CFL bulb in the bulb holder and look at the screen. Do you see the image of the CFL? Is the image inverted or erect?
  3. Now put a filament bulb in place of the CFL. What do you see on the screen? Do you see the image of the rectangular hole? Explain why the two behave differently.
  4. Now open the hinges to widen the rectangular hole. Repeat step-3. what changes do you see.


Figure to be made