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  Physics (NTSE/Olympiad)  

4. Light Refraction

Total Internal Reflection

Total Internal Reflection
Definition : When light travels from a denser medium to a rarer medium and is incident at an angle more than the critical angle for that medium, it is completely returned inwardly in the denser medium. This complete inward return of light is called total (complete) internal (inward) reflection (return).

Fig. Total internal reflection.
CRITICAL ANGLE
The angle of incidence in denser medium for which angle of refraction is 90º, is called the critical angle. It is represented by the symbol C.
µ sin i3 = ua sin 90°

[[Note. More is the value of µ µ lesser will be angle C].
Condition
(i) Light must travel from denser to rarer medium.
(ii) Light must be incident at an angle more than the critical angle for the denser medium.
Merit : In total internal reflection 100% light is reflected, hence images formed are more bright.
In ordinary reflection from mirrors, only 85% light is reflected, rest 15% is either absorbed by mirror glass or transmitted due to poor polish. Images formed by ordinary reflection are less bright.

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