In the figure (a) the light is incident at an angle `theta` (slightly greater than the critical angle). Now keeping the incident ray fixed a parallel slab of refractive index `n_3` is placed on surface AB.(fig b)

Options

A. total internal reflection occurs at AB for `n_(3)=n_(2)`
B. total internal reflection occurs at AB for `n_(3) gt n_(1)`
C. the ray will return back to the same medium for alll values of `n_(3)`
D. total reflection occurs at CD for `n_(3) lt n_(1)`

Answer

C

Explanation

for combination of `n_(1)` and `n_(2)` crtical angle is `theta` when be places parallel slab of refractive index `n_(3)(n_(3) lt n_(1))`. Then crtical angle decreases and T.I.R. takes place for any value of `n_(3)` the ray will be return back in the medium `n_(2)` and reflection takes place at the surface CD.