Total Internal Reflection
3 marks · EasyA triangular glass prism is used in a periscope. The refractive index of the glass is 1.50.
Calculate the critical angle for light inside this prism. Give your answer to 2 decimal places.
Explain why light striking the prism face at 45° is totally internally reflected.
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) 41.81° degree
(b) 45° is greater than the critical angle, so total internal reflection occurs.
Common mistakes: Using wrong equation.; Incorrect substitution.; Not comparing angles properly.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses correct formula: sin c = 1/n with n = 1.50 (follow-through)
A1 (a) · 1 mark · Calculates value: c = 41.81° to 2 decimal places. (follow-through)
B1 (b) · 1 mark · States that total internal reflection happens because the incident angle is greater than the critical angle.
Worked solution
a) Use sin c = 1/n:
b) Since 45° > 41.81°, total internal reflection will occur for light incident at 45°.