Critical angle:
When light refracts from a denser to a rarer medium, the angle of incidence for which the angle of refraction is 90∘ is known as the critical angle.

Total internal reflection: When light refracts from a denser to a rarer medium, if the angle of incidence is greater than the critical angle, the light is reflected back to the denser medium. This phenomenon is known as total internal reflection.
Relation between critical angle and refractive index:
Refractive index of denser medium =µ‌denser ‌
Refractive index of rarer medium =µ‌rarer ‌
Critical angle =θc
‌∴‌ denser µ‌rarer ‌=‌

sin‌θc

sin‌90∘

‌‌

µ‌rarer ‌

µ‌denser ‌

=sin‌θc
‌‌ or, ‌‌‌‌

µ‌denser ‌

µ‌rarer ‌

=‌

1

sin‌θc

‌
If the rarer medium is air, then µ‌rarer ‌=1
∴‌‌µ=‌

1

sin‌θc