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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 for a given value. This angle of incidence is called the critical angle.
![Screenshot 2021-07-30 100928.png](https://resources.cdn.yaclass.in/26307f91-9b84-401b-9ad2-d27fbf039da4/Screenshot20210730100928.png)
Refraction of light from denser to rarer medium
Critical angle:
The angle of incidence at which the angle of refraction is , is called the critical angle.
At this angle, the refracted ray grazes the surface of separation between the two media.
![Screenshot 2021-07-30 100957.png](https://resources.cdn.yaclass.in/50974704-ec75-4ced-a83a-62a4fb3fc574/Screenshot20210730100957.png)
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 \(r\) \(>\) . This is called total internal reflection.
![Screenshot 2021-07-30 101010.png](https://resources.cdn.yaclass.in/c26adaa4-789d-438a-8d37-8c9da42bfac9/Screenshot20210730101010.png)
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.