Name: Formation of Rainbow
Uploaded: 2019-07-19
Duration: 4 min 3 s
Description: Our joy knew no bounds when we used to see a rainbow in the sky in our childhood days. Kids! Do you know how a rainbow is formed? You can learn from this video.

Question

Define critical angle. Explain total internal reflection using a neat diagram.

Accepted Answer

The angle of incidence corresponding to an angle of refraction $90 °$ is called the critical angle for the given pair of media. In other words, if angle of incidence of light, when travelling from a denser medium to rarer medium, is grater than critical angle, total internal reflection takes place.

Consider the following figure.

We know that when a light ray enters from a denser medium to rarer medium, after refraction, in bends away from the normal. Consider the ray ${AO}_{1} B$ . The incident ray ${AO}_{1}$ is partially reflected $(O_{1} C)$ and partially transmitted $(O_{1} B)$ or refracted. Note that the angle refraction $(i)$ is greater than that of angle of incidence $(r)$ . As the angle of incidence increases, the angle of refraction also increases.

Now consider the incident ray ${AO}_{3}$ . In this case, the angle of refraction is $90 °$ . The refracted ray is bent so much away from the normal that it grazes the surface at the interface between the two media. The corresponding angle of incidence is termed as critical angle. This is shown by the ray ${AO}_{3} D$ . If the angle of incidence is increased still further, refraction is not possible, and the incident ray is totally reflected. This is called total internal reflection.

Define critical angle. Explain total internal reflection using a neat diagram.

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