Total Internal Reflection Made by: Kallol Mozumdar Class: X-A Roll no. 11

Previous knowledge Before knowing about total internal reflection, lets revise a little about refraction so that it’s easier for us to understand. Refraction of light is the phenomena of light bending when travelling from one medium to another.

There are two cases of refraction: 1. Light travelling from optically rarer to optically denser medium. As you can see the angle of incidence is greater than the angle of refraction. That’s because the speed of light in denser medium is slow in comparison to that in rarer medium.

2. Light travelling from denser to rarer medium. As you can see that the angle of incidence is less than angle of refraction here. That’s because the speed of light increases when it travels from the denser to rarer medium.

Internal reflection: When light travels from denser to rarer medium at the interface, it is partially reflected back into the same medium and partly refracted to the second medium. This is called the internal reflection.

We know that the refracted ray of light travelling from denser to rare medium moves away from the normal ray, and the “angle r > angle i ”. So what if we increase the angle of incidence?

As we increase the angle of incidence a point comes when the refracted ray grazes over the surface at the interface between the two medium making an angle of 90° with the normal. In this case the angle of incidence is called critical angle. What if we increase the angle of incidence more!!!!!!

When we increase the angle of incidence more than critical angle instead of refracted any more it directly reflects. This phenomena is called the total internal refraction.

Total internal reflection in day to day life: The most common example in daily life is ‘mirage’ in deserts. On the hot summer days, people in deserts very often seem to hallucinate about water source. This happens due to total internal reflection. So hot temp. causes the air near the ground to heat up and rise above creating a low density level there. This difference of density causes the light to refract, & sometimes even reflect.

As you see in the diagram, when the light reflects, it actually seems like bending & the image becomes inverted. Since we cannot see underground, we see it closer to us. So whenever you see a mirage remember that your are actually pretty close to it so don’t give up!!!

Optical fibres: Optical fibres are nowadays used to transmit audio and video signals through long distances. Optical fibres are fabricated with high quality composite glass/quartz fibre consists of a core and cladding. When the light ray enters the fibre from one end it undergoes repeated total internal reflection and finally comes out from the other end.

Activity: ( see it yourself) Materials required: A glass of saturated solution of salt water, a laser pointer Procedure: Take the glass full of saturated salt solution ( Saturated salt solution because it would help in tyndall effect) and hold the laser pointer in the manner shown in the figure.

Now turn the pointer on and keep on tilting the pointer.

A point would come when the ray won’t come out of the top of the glass. At this point notice the water carefully. You will see the total internal refraction of the light.

These are the pictures of the activity done by me, I used a small LED torch instead of a laser pointer. You can see the light reflecting and coming out from the other side, despite of the angle of the torch -- In a super saturated solution

I found this topic very interesting and tried these experiments, I hope even you would grow interest on this topic after watching this PPT & would try these experiments………………… THANK YOU