A glass slab consists of thin uniform layers of progressively decreasing refractive indices RI (see figure) such that the Rl of any layer is `mu - m Deltamu` . Here ` mu` and `Delta mu` denote the Rl of `0^(th)` layer and the difference in RI between any two consecutive layers, respectively. The integer m = 0,1,2,3... denotes the numbers of the successive layers. A ray of light from the `0^(th)` layer enters the `1^(st)` layer at an angle of incidence of `30^@`. After undergoing the `m^(th)` refraction, the ray emerges parallel to the interface. If `mu = 1.5 and Delta mu = 0.015, ` the value of m is

Options

A. 20
B. 30
C. 40
D. 50