State the condition required for total internal reflection.

Hint: Since we know that total internal reflection is the phenomena in which the reflection of the entire incident light is off the boundary. We will see the two conditions which are required to fulfill for a TIR effect to occur.

Complete answer:
Here we know that, Total internal reflection or TIR effect is the phenomenon in which the reflection of the entire incident light is off the boundary. Total Internal Reflection only takes place when the following two conditions are together followed:
 1. Light has to travel from denser medium to rarer medium but not vice-versa.
2. Critical angle of the medium: The Angle of incidence ${\theta }$ must be greater than the critical angle of medium ${\theta _c}$ ​.
The below diagram shows the two conditions of Total Internal Reflection i.e., TIR:


Additional information:
There are some of the places where TIR or total internal reflection is seen. Some of them are mentioned following:
1. In Diamond: Here, when the incident ray falls on the face of the diamond such that the angle formed by this ray is greater than the critical angle. Here, we can observe that the critical value of the diamond is ${23^\circ }$ . This condition is responsible for the total internal reflection in diamond. Therefore the diamond shines.
2. Mirage: Since, mirage is an optical illusion which is responsible for the appearance of the water layer at short distances mainly in a desert. Mirage is also an example of total internal reflection which occurs due to atmospheric refraction.
3. Optical fiber: In optical fire when the incident ray falls on the cladding, it suffers total internal reflection because the angle formed by the ray is greater than the critical angle. Nowadays Optical fibers are used because it has revolutionized the speed with which signals are transferred, not only across cities but across countries and continents making telecommunication one of the fastest modes of information transfer.

Note:
For a total internal reflection both the conditions need to be full fill, not only one. Further we should know the difference between reflection and refraction. Reflection is the change of the direction of wave or light to back to its original medium whereas, in refraction light slightly bends towards the normal when the medium is changed.