Concepts of Physics

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

To trace the light rays to study laws of Reflection and Refraction

Introduction

The laws of reflection and refraction are well known. But to study them one needs to trace light rays. In standard textbook method, pins are fixed in cardboard and the line joining pin heads gives the ray direction. Here we use a much simpler way to do this task.

Apparatus

A working board, a plane mirror, two wooden strips with two screws fitted at the ends, plain paper, scale, pencil, protractor.

Procedure

  1. Fix a plain paper on the working board.
  2. Fix a plane mirror perpendicular to the board in the given slot.
  3. Place one of the two wooden strips on the paper so that the length of the strip is at an angle to the mirror.
  4. Look at the image of this strip. Place the other strip on the paper so that it is in line with the image of the first strip. In this case the two screw heads on the second strip and the image of the two screw heads will be in one line. You can hide three of them behind the one closest to the eye.
  5. Carefully hold a strip and draw a pencil line along the inner edge of the strip. Similarly draw a pencil line along the edge of the other strip. Draw a line along the edge of the mirror.
  6. Extend the lines along the strip edges to meet at the surface of the mirror. One of these may be termed as incident ray and the other reflected ray. Draw a perpendicular from this point to the mirror edge.
  7. Measure the angle of incidence and angle of reflection. Find the difference between them and explain why this difference is.
  8. Try the same with a convex mirror. Are you able to trace the reflected ray corresponding to an incident ray defined by one of the strips. Write the kind of difficulties you face.
  9. Try the same with a concave mirror. Are you able to trace the reflected ray corresponding to an incident ray defined by one of the strips. Write the kind of difficulties you face.

Note

Figure/table to be made