A ray of light incident on a slab of transparent material is partly reflected from the surface and partly refracted into the slab. The reflected and refracted rays are mutually perpendicular. The incident ray makes an angle i with the normal to the slab. The refractive index of the slab is
(a) tan-1 (i)
(b) cot-1 (i)
(c) sin-1 (i)
(d) cos-1 (i)
