A ray of light is incident on a boundary between glass and air.

Which of the following is the refractive index of glass?

A.     \(\frac{{\sin {\theta _1}}}{{\sin {\theta _3}}}\)

B.     \(\frac{{\sin {\theta _1}}}{{\sin {\theta _4}}}\)

C.     \(\frac{{\sin {\theta _3}}}{{\sin {\theta _2}}}\)

D.     \(\frac{{\sin {\theta _4}}}{{\sin {\theta _1}}}\)

D

It was disappointing to see the number of students who incorrectly selected B. This would suggest that they were applying the formula without consideration to the situation. As \(\mu  > 1\), and as \({\theta _4} > {\theta _1}\), the correct answer must be D. It may be advisable to teach Snell’s Law by referring to ‘big’ and ‘small’ angles rather than to angles of incidence and refraction.