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

Play with interference from oil film

Introduction

During rainy days, if mobil oil is spread over wet roads, often you see colored patterns on it. This is due to interference of the waves reflected at the two surfaces of the film. In this experiment there is no definite expected result as the experiment is somewhat without control. So do the experiment and enjoy whatever pattern you get.

Apparatus

A saucer plate, salt solution, oil in a small vessel, a dropper, laser torch, screen

Procedure

1. Put salt solution in the saucer plate. Direct the laser torch on the surface obliquely and collect the reflected light on the screen. You should just see a bright, somewhat broadened spot.
2. Pull some oil in the dropper and gently drop a drop of oil on the solution surface right at the place where laser is falling. Keep looking at the screen, while you put the drop and as the pattern finally settles down. You might see some variation in intensity, fringes and patterns changing with time. This all depends on the thickness of the oil layer which is different at different parts of the drop at a given moment. This is because oil spreads out on water only to a limited extend. Thus the middle part is thick and the sides get gradually thinner.
3. Put another drop on the first drop and see how the pattern forms and develops on the screen. Do it for 2 or 3 more drops.
4. Now using some stick or the dropper tip, mix up the oil and water. Small droplets will form on the surface and will move here and there on water surface. In the process droplets will come under the laser beam and go away. Look at the screen and see how the pattern change as a droplet comes under the laser. Sometimes you may see a complete dark portion moving in the view field and go away. Constructive and destructive interference depending on oil film thickness and the angle from which it comes to the screen, control the pattern.
5. Write whatever you observe in this experiment.