Which of the following will remain constant in the phenomenon of refraction of light?

A ray of light from a denser medium strikes a rarer medium of an angle of incidence $i$. The reflected and refracted rays make an angle of $90°$ with each other. The angle of reflection and refraction are $r$ and $r^{\text{'}}$. The critical angle is

A ray of light travels from an optically denser to rarer medium. The critical angle for the two media is $C$. The maximum possible deviation of the ray will be

A ray of light travelling in water is incident on its surface open to air. The angle of incidence is $\theta$, which is less than the critical angle. Then, there will be

A light beam is traveling from region I to region IV (refer figure). The refractive index in regions I, II, III and IV are $n_0$, and respectively. The angle of incidence $\theta$ for which the beam just misses entering region IV is