Concept
- Total Internal Reflection (TIR): When a ray of light goes from denser to the rarer medium it bends away from the normal and as the angle of incidence in denser medium increases, the angle of refraction in the rarer medium also increases and at a certain angle, angle of refraction becomes
90∘, this angle of incidence is called critical angle (C).
- When Angle of incidence exceeds the critical angle than light ray comes back into the same medium after reflection from the interface. This phenomenon is called Total internal reflection (TIR).
Conditions for TIR:
1. The ray must travel from denser medium to rarer medium.
2. The angle of incidence i must be greater than the critical angle
C.
Explanation:
- On hot summer days, the air near the ground becomes hotter than the air at higher levels.
- The refractive index of air increases with its density. Hotter air is less dense and has a smaller refractive index than the cooler air. Therefore option 2 is correct.
- If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height.
- As a result, light from a tall object such as a tree passes through a medium whose refractive index decreases towards the ground.
- Thus, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle.
- To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object.
- Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called a mirage. Therefore option 4 is correct.
