Total Internal Reflection
Suppose an interface XY separates a rarer medium (a) i.e; air from a denser medium (b) say water shown in fig. below.
Explanation- "O" is a point object in the denser medium. A ray of light starting from "O" & incident normally along OA on XY passes straight along AB. Another ray incident along OA1 deviates away from normal and is refracted along A1B1. Clearly, angle of refraction is greater than the angle of incidence & if increase (talking about angle of refraction) with increase in angle of incidence; for a particular value of i=C, the critical angle, the incident ray OA2 is refracted at 90 degree & goes along interface, along A2B2. When i>C as for the incident ray OA3, the ray goes along A3B3 as if it is reflected from interface XY. This phenomenon is called Total Internal Reflection.
Thus we may define, total internal reflection is the phenomenon of reflection of light into a denser medium from an interface of this denser medium & a rarer medium.
Two essential condition of Total internal reflection:-
- Light should travel from a denser medium to a rarer medium.
- Angle of incidence in denser medium should be greater than the critical angle(C).i,e; i>C
Definition of Critical angle- The angle of incidence in the denser medium upon which angle of refraction in the rarer medium is 90 degree, is called critical angle. it is represented by C & its value depends on the nature of medium in contact.
Relation between refractive index and critical angle
we know from Snell's law
Application
- Refractive index for diamond is 2.42; the calculated critical angle from above relation is 24.4 degree. The diamond is cut suitably such as i>C. Therefore it suffers multiple reflections at various faces and remains within the diamond. Hence diamond is sparkles.
- Optical fiber works on the principle of total internal reflection.
- The phenomenon of Mirage can be explained on the basis of total internal reflection.
- The phenomenon of Looming can be explained on the basis of total internal reflection.
- Total internal reflection is used in many optical instruments like telescopes, microscopes, spectroscopes etc.
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