A ray of light ( R _ 1 ) is incident on a glass slab at an angle...

A ray of light $\left({R}_{1}\right)$ is incident on a glass slab at an angle equal to critical angle of glass-air system. The refracted ray $\left({R}_{2}\right)$ undergoes partial reflection and refraction at other surface of slab. The angle between the reflected ray $\left({R}_{3}\right)$ and refracted ray $\left({R}_{4}\right)$ is