Concepts of Physics

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

Properties of Lens and Mirror Using Parallel Beam


To know the physics of the light box developed at Anveshika.


Light box is a special equipment designed by Anveshika. It has three slits. The light box contains a 12 V bulb. The power to the bulb is provided by a 220 V to 12 V step down transformer. The light from the bulb passes through a lens and comes out as three parallel beams from the slits. We can close a slits by putting a black tape on it.


Light box, mirrors and lenses of different types.


  1. Carefully open the light box. See the light bulb, lens, and three slits.
  2. Now, adjust the bulb (by moving cylindrical rod) so that you get a parallel beam of light. Where is the bulb located with respect to the lens? What is approximate focal length of the lens?
  3. How do you know that beam is parallel? For the beams to be parallel, the distance between the beams should not change with change in distance from the source.
  4. Now, see the effect of convex lens, concave lens, concave mirror and convex mirror on the parallel beam.


The bulb is like a point source of light. We get parallel beam of light when bulb is placed at the focus of a convex lens.