Question

(i) Name the phenomena on which the working of an optical fiber is based.
(ii) What are the necessary conditions for this phenomenon to occur?
(iii) Draw a labelled diagram of an optical fiber and show how light propagates through the optical fiber using the phenomenon.

Answer 1

Hint: Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves.


Complete step-by-step solution:
(i) The optical fiber is based on total internal reflection. Total internal reflection is the phenomenon which occurs when the light rays travel from a more optically denser medium to a less optically denser medium.
Total internal reflection, \[\dfrac{{{n_1}}}{{{n_2}}} = \dfrac{{\sin i}}{{\sin r}}\]
Critical angle, $\theta = \dfrac{{{n_1}}}{{{n_2}}}$
r=the angle of refraction
i=the angle of incidence
${n_1}$=the refractive index in medium 1
${n_2}$=the refractive index in medium 2
$\theta $= the critical angle
(ii) There are two conditions for total internal reflection.
The light ray moves from a denser medium to a less dense medium.
The angle of incidence must be greater than the critical angle.
(iii)

Additional information:
When a primary rainbow is formed, it is due to one total reflection in water droplets. A secondary rainbow is formed by light that underwent two internal reflections, and that is what changes the ordering of the colors.

Note: Mirage is caused due to total Internal Reflection of light by the various layers of air. It is an optical illusion which is responsible for the appearance of the water layer at short distances in a desert or on the road. Mirage is an example of total internal reflection which occurs due to atmospheric refraction.