Waves
4 marks · NormalA ray of light passes from air into a rectangular glass block at an angle of 38°. Inside the block, the angle of refraction is 24°.
Calculate the refractive index of the glass to 2 decimal places.
Explain what happens to the direction of the light as it enters and leaves the glass block.
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) n = sin(38°)/sin(24°) = 0.6157/0.4067 ≈ 1.51
(b) When entering the glass, the light bends towards the normal (slows down); when leaving the glass to air, it bends away from the normal (speeds up).
Common mistakes: Incorrect application of formula; Not identifying direction change
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Substitutes into n = sin(i)/sin(r)
B2 (a) · 1 mark · Correct calculation to 2 dp (accept 1.50–1.52)
B3 (b) · 1 mark · Explains bending towards the normal on entering (slowing down)
B4 (b) · 1 mark · Explains bending away from the normal on leaving (speeding up)
Worked solution
a) n = sin(38°)/sin(24°) = 0.6157/0.4067 ≈ 1.51
b) The ray bends towards the normal when it enters the slower medium (glass) and away from the normal when it exits glass into air.