A ray of light is incident on an air-glass boundary at an angle t? If the angle between the partial reflected ray and the refracted ray is 90°, calculate θ given that the refractive index of glass is 1.50

This question is unpopular as many candidates avoided it. Those who attempted it failed.
The problem was with the trigonometric function of linking the angle of refraction to sin (90- θ)
= Cos θ
The expected answer is:
n = Sin θ
Sin r
= Sin θ
Sin (90- θ)
= Sin θ
Cos θ
= tan θ
Θ= (1.5) = 56.31°