Total Internal Reflection
4 marks · NormalA beam of light travels from water (refractive index 1.33) into air.
Calculate the critical angle for the water–air boundary. Give your answer to 1 decimal place.
Would total internal reflection occur if the angle of incidence in water is 48°? Show your working.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) sin c = 1/1.33 = 0.752; c = arcsin(0.752) = 48.8°
(b) No, because the angle of incidence (48°) is less than the critical angle (48.8°).
Common mistakes: incorrectly finding sin c; confusing air/water order
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses sin c = 1/n.
A1 (a) · 1 mark · Correctly substitutes n = 1.33, finds sin c = 0.752.
A2 (a) · 1 mark · Evaluates c = 48.8°.
B1 (b) · 1 mark · States TIR does not occur since angle (48°) < critical angle (48.8°). (follow-through)
Worked solution
The angle given (48°) is less than the critical angle (48.8°), so total internal reflection does not occur yet.