Concepts of Physics

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

To Visualize a Ray of Light


The phenomenon of light are discussed at school level. Entire discussion is mostly indirect. Students never see the light ray.


Transparent plastic box, laser torch, incense stick.


Take a plastic box with a lid. Open the lid and put some smoke in the box using an incense stick. Close the lid. Keep a laser torch just outside the box. Send laser light into the box from one side. You see the light beam very clearly. This also shows that light travels in a straight line.

Variant: You can sprinkle the dust from the duster used on blackboard.

Variant: Place a glass of tap water in front of a paper screen or wall. Shine a laser pointer (or a powerful torch) through the water. You will only see spots of light on the screen and on the wall of the glass. Now, put a drop of milk in the water and shine light through the water. You will be able to see the path of the light through the water.


What is the role of smoke or dust? How does it makes the laser light visible? The smoke or dust particles scatter the laserlight towards our eyes and hence make it visible.