What is the critical angle of a diamond having a refractive index of 2.42? use air as the second medium. air has the refractive index 1.00.

a. 14.9°
b.24.4°
c.36.6°
d.40.9°

Given:
refractive index: diamond = 2.42 ; air = 1

90° should have been provided in the problem. I encountered similar problem before. 

Using Snell's Law: n1*sin(a) = n2*sin(b)
where:
n1 and n2 are the refractive indexes
sin(a) and sin(b) are the corresponding angles.

n1 = 2.42
n2 = 1.00
b = 90°

n1 * sin(a) = n2 * sin(b)
2.42 * sin(a) = 1 * 1
2.42 * sin(a) = 1
sin(a) = 1 / 2.42
sin(a) = 24.4°  Choice B.