Refraction through a rectangular glass block
Trace a ray of light into a rectangular glass block with optics pins, sight through the far side to find where it emerges, and show the emergent ray is parallel to the incident ray — displaced sideways, but travelling in the same direction. Measure the angle to confirm it equals the original angle of incidence.
WHAT YOU'LL LEARN
- ✓Why a ray bends toward the normal entering a denser medium, and away from it leaving
- ✓Why a parallel-sided block only displaces a ray sideways, without changing its final direction
- ✓Why pins placed further apart give a more accurate ray-traced line
ON YOUR BENCH
- □Ray-trace sheet (plain paper, tied into a booklet at the exam)
- □Rectangular glass or Perspex block, approx. 10 cm × 6 cm × 1.5 cm
- □4 optics pins
- □Pin board (e.g. a cork mat)
- □Protractor
- □30 cm or 50 cm ruler, graduated in mm
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Draw round the block
Place the block on the ray-trace sheet and draw its outline, labelling the corners A, B, C, D. Remove the block before drawing anything else — pencil marks under glass are invisible and inaccurate.
- 02
Draw the normal
Draw a line NL perpendicular to side AB, 2.0 cm from corner A, continuing it straight through side CD. Label the point Q where it crosses AB.
- 03
Draw the incident ray and place P1, P2
Draw a line PQ meeting the normal at Q at your chosen angle of incidence (30° first). Push two optics pins, P1 and P2, into the line PQ — more than 5.0 cm apart, so a small error in either pin's position barely changes the line's angle.
- 04
Replace the block and sight through it
Put the block back over its outline. Looking through side CD, line up your eye with the images of P1 and P2, then place pins P3 and P4 so all four pins — P1, P2 and the images of P1, P2 — appear exactly one behind the other.
- 05
Trace the emergent ray and measure θ
Remove the block and the pins. Draw a straight line through the P3–P4 pinholes, continuing it back until it meets the normal at a point E. Measure the acute angle θ between this line and the normal — it should equal your angle of incidence, within a couple of degrees.
- 06
Repeat at a second angle
Repeat the whole trace at i = 45° for a second i–θ data point, then plan further angles to test the relationship (spread over at least 30°, e.g. 20°, 40°, 60°, 70°).
What you should see
- →The refracted ray inside the block bends toward the normal (entering the denser glass); the emergent ray bends back away from the normal by the same amount leaving it, so it ends up parallel to the incident ray — just shifted sideways.
- →θ measured on the exit side should equal the angle of incidence i to within about ±2°, the usual tolerance for hand-drawn ray traces.
- →Pins placed close together (5 cm or less) scatter the measured angle noticeably from one attempt to the next; pins spread further apart give a far more repeatable angle.
| i /° | 30 | 45 |
|---|---|---|
| θ /° | 29 | 44 |
⚠ BEFORE YOU START
- •Optics pins are sharp — push them into the pin board vertically, not toward your hand, and count all four back out at the end.
- •Keep your eye a sensible distance from the block while sighting through it; don't lean directly over the pins.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.