Reflection, refraction and refractive index n = sin i / sin r – IGCSE Physics exam questions with mark scheme

1Grade 4–54 marks

A ray of light passes from air into a glass block, as shown.

(a) Calculate the refractive index of the glass. [3]

(b) Explain why the ray changes direction as it enters the glass. [1]

(a) $n = \dfrac{\sin i}{\sin r}$ accept $\sin i = n \sin r$; mark may be implied by correct substitution(1)
(a) $n = \dfrac{\sin 40^\circ}{\sin 25^\circ}$(1)
(a) 1.52 allow 1.5; no unit; correct answer with no working scores 3(1)
(b) the light slows down / its speed decreases (as it enters the glass); accept 'its speed changes'; ignore 'glass is denser' alone(1)
Answer: (a) 1.52 (b) light slows down as it enters the glass
Examiner tip: Angles are measured from the normal, not from the surface — using 50° and 65° gives the wrong index.

2Grade 6–76 marks

A student wants to measure the refractive index of a rectangular glass block.

She has a ray box, a protractor, a ruler, a pencil and a sheet of paper.

Describe an experiment she could do to find the refractive index. [6]

place the block on the paper and draw around it (outline the block) accept mark the position of the block(1)
use the ray box to direct a (narrow) ray of light at the side of the block at an angle to the normal reject ray along the normal(1)
mark the incident ray and the emerging ray (e.g. with crosses / dots / pencil lines), remove the block and join the entry and exit points to show the ray inside the block(1)
draw the normal (at 90° to the surface) where the ray enters the block(1)
measure the angle of incidence and the angle of refraction from the normal with the protractor(1)
calculate $n = \sin i \div \sin r$ allow plot sin i against sin r, gradient = n. Max 6: if one of the points above is missing, a description of repeating for different angles of incidence and calculating a mean n may be credited in its place(1)
Answer: Draw round block, trace incident and emergent rays, draw normal, measure i and r, n = sin i / sin r, repeat and average.
Examiner tip: Measure both angles from the normal — and say how you get the ray inside the block (join entry and exit points).

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