Physics 11–12 · Year 12

Diffraction of laser light through slits, a thin wire and a grating (qualitative) (syllabus practical)

Module 7: The Nature of Light (Light: Wave Model)

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Light passing an edge or a narrow opening spreads and forms a pattern of bright and dark bands, which only a wave model explains.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

laser beam entering an eye directly or by reflection

Controls

Class 2 lasers only, below eye level, beam stop behind the screen, no reflective jewellery, never look into the aperture

Note

Lasers: follow Science ASSIST SOP Use of lasers in schools (https://asta.edu.au/resource/sop-use-of-lasers-in-schools/) and record the activity in RiskAssess (https://www.riskassess.com.au/).

What you need

Class 2 red laser (650 nm) and a green laser (532 nm) on a stand (the NSW Department of Education diffraction learning sequence uses a 650 nm red laser)
Slide set: single slits of 0.04 to 0.2 mm, double slits, a thin wire, a diffraction grating, a circular aperture
White screen 2 m away, ruler, darkened room, beam stop

How to do it

Complete the predict-explain-observe-explain (PEOE) table from the learning sequence for each aperture before switching on.
Shine the red laser through the 0.1 mm single slit and sketch the pattern; note the wide central band and the fainter side bands.
Swap to a narrower slit: the pattern widens. Swap to the green laser: the pattern narrows.
Try the thin wire (same pattern as a slit of the same width), the double slit (fine fringes inside the single-slit envelope) and the grating (sharp widely spaced spots).
Record how each change of wavelength or aperture altered the spacing.

What you should see

A 0.10 mm slit with a 650 nm laser gives a central bright band 26 mm wide on a screen 2.0 m away (first minima at 0.37 degrees); the 532 nm laser narrows it to 21 mm; halving the slit width doubles the width. The double slit shows fine fringes inside the same envelope and the grating throws bright spots tens of centimetres apart.

What changes

What you change: slit width and wavelength
What you measure: width and spacing of the pattern
What you keep the same: screen distance
laser alignment

Common misconceptions

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

Light through a narrow slit makes a narrow line; the narrower the slit, the wider the spread.
Diffraction only happens with very special apertures; every edge diffracts, the effect is just small when the opening is large.

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 2027PH12-14 PH11/12-1 PH11/12-3
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-03 PY-12WS-01
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities