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Question
Answer the following question.
Under what conditions are total internal reflection possible? Explain it with a suitable example.
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Solution
Conditions for total internal reflection:
- The light ray must travel from denser medium to rarer medium.
- The angle of incidence in the denser medium must be greater than critical angle for the given pair of media.
Total internal reflection in optical fibre:
- Consider an optical fibre made up of the core of refractive index n1 and cladding of refractive index n2 such that, n1 > n2.
- When a ray of light is incident from a core (denser medium), the refracted ray is bent away from the normal.
- At a particular angle of incidence ic in the denser medium, the corresponding angle of refraction in the rarer medium is 90°.
- For angles of incidence greater than ic, the angle of refraction becomes larger than 90° and the ray does not enter into the rarer medium at all but is reflected totally into the denser medium as shown in the figure.
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