Light travels inside a glass prism. The refractive index of the glass is 1.50.
(a) Calculate the critical angle for the glass. [3]
(b) A ray inside the glass hits the glass–air boundary at an angle of incidence of 50°.
Describe and explain what happens to the ray. [2]
- (a) $\sin c = \dfrac{1}{n}$(1)
- (a) $\sin c = \dfrac{1}{1.50}$ or $c = \sin^{-1}(0.667)$(1)
- (a) 42° allow 41.8°(1)
- (b) (total internal) reflection / the ray is reflected back into the glass(1)
- (b) because the angle of incidence (50°) is greater than the critical angle allow ecf from (a)(1)
- Answer: (a) 41.8° (b) total internal reflection because 50° > critical angle
- Examiner tip: Total internal reflection needs the ray to be going from glass towards air AND the angle of incidence to exceed the critical angle — state the comparison.