Concepts of Physics

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

How does the reflected intensity vary with the angle of incidence?

Objective

How does the reflected intensity vary with the angle of incidence.

Introduction

When a beam of light falls on the surface of a transparent material, part of it gets reflected and part of it gets transmitted. What fraction of the incident beam intensity gets reflected and what fraction gets transmitted? It depends on the refractive indices of the two media, the angle of incidence and also the state of polarization. In this experiment you will explore the dependence qualitatively on the material and the angle of incidence.

Apparatus

A vessel, a glass slab, a laser fitted in a stand, a white screen that can be tilted.

Procedure

  1. Put water in the vessel. Arrange the laser in the stand so that it can give light falling obliquity on the surface of water. Arrange the screen so that it receives the reflected light almost perpendicularly.
  2. Switch on the laser. Hopefully you will see a bright spot of reflected light on the screen. If you find it very faint and you are sure that the battery of the laser in not depleted, slightly rotate the laser torch about its own axis, without changing the direction of the light falling on water.
  3. Observe the intensity of the spot on the screen. Now without disturbing the laser or the screen, remove the vessel and put the glass slab in its place.
  4. Question: Did the intensity decrease, increase or remain same?
  5. With glass slab in place, change the angle of incidence by changing the direction of the laser beam. Adjust the screen to receive the beam nearly perpendicularly. Do it for \(\theta\) close to 10 degree-15 degree, 40 degree-50 degree and then to 70 degree-80 degree. See in what way intensity changes and enter your observations in the table.
  6. Fix the laser torch to send light on the slab at an angle of about 50 degree-60 degree. Rotate the torch at its own place without changing the direction of the incident light. Can you get ZERO INTERNAL REFLECTION?

Note

Figure to be made