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

# Double Refraction in Water Bottle

## Objective

See double refraction in water bottle.

## Introduction

Double refraction is a property of anisotropic material which offers different refractive indices for different orientations of E-field of light (birefringence). In such cases, often an incident beam splits in two parts on entering the medium. In this experiment it appears that a beam incident on water in a bottle splits into two on entering the water mass. But it is certainly not the case of double refraction in its true meaning.

## Procedure

1. Take water in the bottle up to about 3/4 of height. Put one or two drops of dettol in this water. Place the bottle on a horizontal surface.
2. Take a laser torch and let the laser fall on the water surface obliquely. It will refract in water and the path will be visible from the side.
3. Adjust the position of the laser so that it falls right at the junction of the glass surface and the water surface. Do fine adjustments and you will see two refracted beams!

Question: Are both the beams straight? Draw a diagram showing how the two beams appear.

## Discussion

The water surface near the glass surface gets curved due to surface tension. The angle of incidence for the part of the beam on this part of water surface is different from what the beam makes with the flat part of the water surface. This gives appearance of two refracted beams.