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 refreactive index of the slab is
A. `tan^(-1)(i)`
B. `cot^(-1)(i)`
C. `sin^(-1)(i)`
D. `cos^(-1)(i)`
