refractive index

Diagram shows a light ray entering a semicircular glass block and being refracted.
Rajah menunjukkan satu sinar cahaya memasuki satu bongkah kaca semibulatan dan dibiaskan.

What is the refractive index of the glass block?
Berapakah indeks biasan bongkah kaca itu?

A     1.50
B     1.58
C     1.66
D     1.72

Answer : C

The critical angle is the greatest angle of incidence in the optically denser medium for which the angle of refraction, r = 90^o

Refractive index, n = 1

                                            sin c