Optics of Total Internal Reflection: From Diamonds to Mirages

Swimming is certainly a fun activity. We all love playing with water and while we dive there are so many things we explore. We often see mirror images of things in the water. Well, not everybody swims. Let’s talk about the mirage.

There’s a deceptive presence of water on the road that we can see from far but when we get closer, the road is all clear.

What is the cause of this? The phenomenon behind is called total internal reflection.

The Phenomenon of Reflection

We all know that light travels and it falls on things. When it bounces back the process is called the reflection of light.

However, sometimes light doesn’t reflect on hitting a surface and it rather enters it and changes its direction. This process is called refraction. This happens when light changes its medium. For example, it travels from the air and enters into water or vice versa.

The Angle of Incidence and Refraction

When light enters into a different medium, the angles at which it strikes the surface and changes its direction are called the angles of incidence and angles of refraction respectively. Any change in the angle of incidence also changes the angle of refraction.

Total Internal Reflection

When light travels from a denser medium to a rarer medium, refraction occurs till the point the angle of incidence reaches an angle called a critical angle. The critical angle for every medium is different. For example, the critical angle for glass is different from the critical angle for water.

So, what happens to the light when it stops refracting when the critical angle is crossed by it? Or what happens when the angle of incidence is greater than the critical angle? At this point, light reflects back into the same medium without entering the other medium. This process is called the total internal reflection.

The Two Rules of Internal Reflection

All in all, there are two rules for total internal reflection.

1. The ray of light should be traveling from a denser medium to a rarer medium.
2. The value of the angle of incidence must exceed that of the critical angle.

Thus, the surface of water looks like a mirror because there’s a total internal reflection going on. This phenomenon plays a very important role in optical fibers as well.

Optical fibers are used in communication via the transmission of light. They carry signals at a much faster pace as compared to the normal electric cables because they employ laser light which reflects back to the fiber walls when the incident ray turns out to be greater than the critical angle. 

Real Life Instances involving Total Internal Reflection

• Endoscope, a medical instrument which is employed for exploratory diagnostics and used in surgeries involves fiber tubes in its construction. The fiber tubes emit light which falls on the organs and then reflects back to the lens through which the doctor gets the image of the organ. This reflection caused thereby is the total internal reflection.

• Binoculars, periscopes and prisms are some other examples that involve the phenomenon of total internal reflection in their operation.

• What about diamonds? Why do they shine so much? It’s their make which allows the light to reflect internally for multiple times as the refractive index of diamond is exceptionally high. We must remember, the greater the refractive index, greater will be the number of total internal reflections because of the critical angle being so small.