Light is refracted away from the normal when it travels from a denser medium into a rarer medium. The angle of refraction increases in lockstep with the angle of incidence in the denser medium, eventually reaching a maximum of r=90° for a given value. This angle of incidence is called the critical angle.
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Refraction of light from denser to rarer medium
Critical angle:
The angle of incidence at which the angle of refraction is 90°, is called the critical angle.

At this angle, the refracted ray grazes the surface of separation between the two media.
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Critical angle
The refraction is not possible when the angle of incidence exceeds the critical angle value. The ray is completely reflected back to the same medium because \(r\) \(>\) 90°. This is called total internal reflection.
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Total internal reflection
Conditions to achieve total internal reflection:

In order to achieve total internal reflection, the following conditions must be met.
  • The light must travel from a denser medium to a rarer medium. (Example: From water to air).
  • The angle of incidence in the denser medium must be greater than that of the critical angle.