Hey there
The two necessary conditions for total internal reflection are
1. Light should be moving from denser medium to rarer medium. Like from glass to air.
2. The angle of incidence should be greater than critical angle which can be find if you knows the refractive index of the medium.
Good luck
Question : A light ray travelling from denser to rarer medium at an angle of incidence more than the critical angle of the concerned media pair suffers:
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