Question 3 — Refraction through a rectangular glass block
Trace an incident ray into a glass block with optics pins, find the emergent ray by sighting through it, and measure the equivalent angle on both sides of the normal.
RUN IT ON THE BENCH
Work through it, then fill in your answers below.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 3 — refraction apparatus
- ray-trace sheet(1)
- transparent block(1)
- optics pins(4)
- pin board(1)
- protractor
- 30 cm or 50 cm ruler
- string or treasury tag
THE BENCH — RAY-TRACE SHEET, GLASS BLOCK & OPTICS PINS
More than 5.0 cm apart — a small pin wobble barely moves the line.
The dashed preview shows the ideal, noise-free ray. Sighting "commits" a real trial — with the same small pin-placement wobble a hand has on paper.
Why does the ray come out parallel to how it went in?
AB and CD are parallel faces. Going in, the ray bends toward the normal (slowing down in the denser glass); coming out through the parallel face it bends away from the normal by exactly the same amount it bent going in. The two bends cancel, so the emergent ray travels in the same direction as the incident ray — just shifted sideways by the trip through the glass. That's why θ, measured against the normal on the far side, should equal i.
Why do closer pins give a worse angle?
A pin can only be placed to within a fraction of a millimetre by eye. Over a short baseline that error is a large fraction of the distance, so the line you draw through the two pins can tilt several degrees off the true ray. Spread the same two pins further apart and the same placement error is a much smaller fraction of the baseline — the line's angle is far steadier. That's the whole reason the mark scheme wants P1/P2 "more than 5.0 cm apart".
YOUR TASKS · 0/4
- ✓Sight through the block at i = 30° and record θ
- ✓Keep P1/P2 more than 5.0 cm apart
- ✓Read off θ from your trace
- ✓Repeat the trace at i = 45°
YOUR TRIALS
Sight through the block to record your first trial.
To produce the most accurate ray-trace, a student places pins P1 and P2 ...
YOUR ANSWERS
Place the block on the ray-trace sheet, draw and label its outline ABCD. Remove the block and draw the normal NL to side AB, 2.0 cm from A, continued through side CD. Label the point Q where NL crosses AB.
Draw line PQ at angle of incidence i = 30° to the normal. Place pins P1 and P2 on PQ. Replace the block and, sighting through side CD, place pins P3 and P4 so that P3, P4 and the images of P1, P2 appear exactly one behind the other. Label P1–P4 and remove the pins.
Draw a line through P3 and P4, continue it until it meets NL, and label that point E; label the other end F. Measure the acute angle θ between EF and the normal.
Repeat the procedure in (b) and (c) using an angle of incidence i = 45° to obtain a new value of angle θ.
Tick one box to complete the sentence: 'To produce the most accurate ray-trace, a student places the pins P1 and P2 ...'
A student plans to investigate the relationship between angle i and angle θ, taking more sets of readings. List suitable values of angle i that the student can use.