Let the refractive index of a denser medium with respect to a rarer medium be n_12 and its critical angle be θ_C. At an angle of incidence A when…

Let the refractive index of a denser medium with respect to a rarer medium be $\displaystyle \mathrm{n}_{12}$ and its critical angle be $\displaystyle \theta_{\mathrm{C}}$. At an angle of incidence A when light is travelling from denser medium to rarer medium, a part of the light is reflected and the rest is refracted and the angle between reflected and refracted rays is $\displaystyle 90$°. Angle A is given by :