Total Internal Reflection

 Have you ever experienced this?

When you are out walking on the footpath, and  you feel your hair is not proper but don't have a mirror, You look onto the glass windows of a car parked nearby and see yourself ." Oh this could work as a mirror too" and start correcting your messy hair, when suddenly the person inside the car rolls down the window and smiles. That embarrasing laugh you gave back ? 

Or when you walk across shops or cafes with glass panes and at times you see yourself in them? 

or the mesmerizing feeling when you see a diamond sparkle effortlessly? 

In the first 2 examples, We know that these windows and glass panes are transparent, which is they should refract the light and let it pass through them, But how are they reflecting them back?? 

Due to the phenomena called TOTAL INTERNAL REFLECTION! 

To understand this concept , Let's take an example of a glass slab, the light getting refracted through it's second surface, Like this 👇

Now, If we keep increasing the angle of incidence, the angle of refraction keeps moving away from the normal.( Denser to rarer , away from the mirror) 

At one point the refracted ray become almost parallel to the surface of the interface , 90 degrees to the  normal  

 

The Incident angle which causes the Refraction angle to become  90° is called Critical Angle.

So, using Snell's Law :- ( If you haven't read my previous posts about Snell's Law Click here!)

μGlass sin i = μAir sin r 

(sin 90° = 1and if we consider Glass as medium 1 and Air as medium 2)

μ1 sin i = μ2

sin critical angle =  μ2 / μ1 

So if the incident ray is either equal or greater than the critical angle, the light doesn't refract but is transmitted back into the medium of origin. 

To sum it up ,condition required for Total Internal Reflection to occur:-

1) The ray of light should move from denser to rarer medium

2) Angle of incidence should be greater than or equal to Critical angle

NOTE:- Total internal reflection is considered to be a special type of reflection as here, there is no dissipation of energy , in usual reflection cases, a bit of the energy is dissipated during reflection causing the energy while incidence > energy after reflection.

But here , energy while incidence = energy after reflection 

Total internal reflection has a lot of applications in real life ( including putting you into that embarassing moment in the start 😅) 

1) It causes the sparkling effect in diamonds

In a Diamond the refractive index is 2.5 , hence when the light falls on it in the angle of 24°, it reflects the light back into the first medium. The diamond makers cut the diamond in such a way that the angle of incidence is usually greater than the critical angle hence causing the awestruck sparkle.

2) It is used in Optical fibres ( used to transmit internet , without which you won't even be able to read this blog )

3) And also plays a major role in the formation of rainbows 

Now we know that physics is everywhere! From the skies to the vast oceans and from expensive jewellery to embarrasing moments in everyday life. Physics is to be both blamed and praised😝