Total Internal Refraction

Definition

It is the phenomenon in which a ray of light travelling from denser to rarer medium is reflected back into the incident medium(denser medium)if the angle of incidence is greater than the critical angle.

This only happens when light travels from optically denser to rarer medium.

Conceptual Points

·         When a light ray travels from denser to rarer medium, the refracted ray bends away from the normal. The angle of refraction is greater than the angle of incidence.

·         The angle of refraction increased with increase in the angle of incidence. For a particular angle of incidence, the angle of refraction becomes equal to 90o .This angle of incidence is called critical angle.

·         When the angle of incidence exceeds the critical angle, the incident light ray instead of being refracted gets reflected back into the rarer medium(incident medium).

This phenomenon is called total internal reflection

The conditions under which total internal reflection occurs:-

(a) The light ray should travel from denser medium to rarer medium.

(b) The angle of incidence in the denser medium (incident medium)must be greater than the critical angle.

Applications Of Total Internal Reflection

Hot Mirage ( Mirage) or Inferior Image :-

It is an optical illusion due to which a traveller in a desert or a very hot place gets the impression of presence of a pool of water from a distance.

Explanation :-

In deserts the air near the ground is hotter and of lower density. Air at a height above the ground is relatively less hot and of higher density. Light rays starting from a tree passes successively from denser to rarer medium. The refracted rays bend away from the normal and the angle of incidence gradually increases. When angle of incidence goes above the critical angle, total internal reflection occurs and the light ray is reflected back. Hence a virtual image of the object is seen by the person, which conveys the illusion of reflection of object in water.

COLD MIRAGE ( LOOMING) or SUPERIOR MIRAGE:-

It is an optical illusion occers in a cold due to which a traveller in a ver cold place gets the impression of objects ( ship) from a large distance floating in the air.

In cold or polar regions, air at lower level is comparatively cooler (denser) than the air at higher levels (rarer). Hence a beam of light coming from a ship travels from denser to rarermedium and the angle of incidence goes on increasing with each layer of air. When the angle of incidence exceeds the critical angle, the light ray gets reflected back due to total internal reflection. Hence the observer looking straight sees the virtual image of the ship appears to be floating in the air.This optical illusion is called LOOMING or Superior Mirage.