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Define critical angle.

Last updated date: 15th Aug 2024
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Hint: Going through the concept of total internal reflection, we can understand the definition of critical angle. Whenever refraction takes place, partial reflection is also accompanied with it and it increases with the increase in angle of incidence.

In optics, the smallest angle of incidence that causes the total internal reflection of light is known as the critical angle. Total internal reflection refers to the complete reflection of a light ray at the interface of two mediums with different refractive index and the light ray approaches from the denser medium and if is incident on the interface with an angle equal to or greater than critical angle gets reflected back instead of undergoing refraction.

Suppose the light ray is incident through a medium of refractive index $n_1$ and hit the interface of another medium with refractive index $n_2$, such that $n_2 \le n_1$, then the critical angle is given by $\theta_c =sin^{-1}\left(\dfrac{n_2}{n_1}\right)$.

In the diagram below, we can see that, at the critical angle, the light rays gazes upon the surface of the interface and as we increase the angle further, total internal reflection takes place.