Physics 11–12 · Year 11

Refraction and total internal reflection with a semicircular perspex prism and ray box (syllabus practical)

Module 3: Waves and Thermodynamics (Ray Model of Light)

Practical, model not builtLow risk

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The idea

Light bends at a boundary because its speed changes, and the ratio of the sines of the angles is the refractive index, which also sets the critical angle.

What you need

Ray box with single-slit mask and a 12 V supply, semicircular perspex (PMMA) prism, rectangular glass slab
A4 paper, protractor, sharp pencil, ruler; darkened room
Optional: rectangular container of water for a second medium

How to do it

Trace the semicircular prism and mark its centre and normal; aim the ray at the centre of the flat face from the curved side so it enters undeviated.
Set angles of incidence of 10, 20, 30, 35 and 40 degrees inside the perspex (all below the 42 degree critical angle) and mark the emerging ray each time; measure the angle of refraction in air.
Increase the internal angle until the refracted ray skims the surface; record the critical angle, then show total internal reflection beyond it.
Plot sin(angle in air) against sin(angle in perspex); the gradient is the refractive index (this is the NSW Department of Education Module 3 guide, Activity 9).
Repeat with the glass slab and the water container to compare indices.

What you should see

The sine plot is a straight line through the origin with gradient near 1.49 for PMMA (1.491 at 589 nm). A ray at 40 degrees in air refracts to 25.5 degrees inside the perspex; the critical angle for PMMA is 42.1 degrees, for crown glass (n = 1.52) 41.1 degrees and for water (n = 1.33) 48.8 degrees. A 1 degree error in the angle read inside the perspex at 30 degrees changes n by about 0.05 (1.54 rather than 1.49), so n is taken from the gradient of all the points rather than from one reading.

What changes

What you change: angle of incidence
What you measure: angle of refraction
What you keep the same: same material
ray aimed at the centre of the flat face
single narrow ray

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Light bends towards the normal because the perspex pulls it; it bends because it slows on entering the denser medium.
Total internal reflection can happen going from air into glass; it needs light travelling from the higher-index medium towards the lower.

Safety card

Low riskLearners carry it out

Hazards

hot ray-box lamp
dark room trip hazard

Controls

12 V lamp, switch off when not in use
clear the floor before darkening

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-10 PH11/12-3 PH11/12-4
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-02 PY-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities