Total Internal Reflection
5 marks · NormalA student uses a prism made from glass with refractive index 1.52. She shines a ray of light at 49° to the normal onto the surface between glass and air.
Calculate the critical angle for this glass–air boundary. Give your answer to 1 decimal place.
Will the ray undergo total internal reflection at the boundary? Explain your answer with a calculation or comparison.
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.
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(a) sin c = 1/n = 1/1.52 = 0.658; c = 41.1°
(b) Yes, total internal reflection occurs because the angle of incidence (49°) is greater than the critical angle (41.1°).
Common mistakes: incorrectly finding critical angle; failing to compare correctly
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses correct formula for critical angle: sin c = 1/n.
A1 (a) · 1 mark · Substitutes n = 1.52, obtains c = 41.1° (accept 41°–41.2°).
B1 (b) · 1 mark · States comparison between angle of incidence and critical angle.
B2 (b) · 1 mark · Explains total internal reflection will/must happen since angle of incidence > critical angle.
A2 (b) · 1 mark · Correct conclusion: Yes, total internal reflection will occur. (follow-through)
Worked solution
The angle of incidence is 49°, which is greater than the critical angle. Therefore, all the light is totally internally reflected at this boundary.