Physics 11–12 · Year 12

Measuring a laser's wavelength with a double slit and a diffraction grating (quantitative) (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

Bright fringes appear where the path difference from two sources is a whole number of wavelengths, so the fringe geometry gives the wavelength.

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 and its bright grating orders reaching an eye

Controls

Class 2 lasers only, below eye level, beam stop, no reflective surfaces in the beam

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 (nominal 650 nm) on a stand, double-slit slide with a stated separation (for example 0.25 mm), diffraction grating of 600 lines per mm
Screen at 2.00 m, metre rule and a ruler graduated in 0.5 mm, darkened room, beam stop

How to do it

Double slit: mark the centres of ten adjacent bright fringes on the screen and measure the span; divide by nine for the fringe spacing.
Measure the screen distance L; compute lambda = (fringe spacing) x d / L.
Grating: measure the distance from the central spot to the first-order spots on both sides; for the second order move the screen to 0.500 m, because at 2.00 m it lies 2.49 m from the centre; compute the angles with tan and the wavelength from d sin(theta) = m lambda.
Repeat each measurement three times and report the wavelength with its uncertainty.

What you should see

With d = 0.25 mm and L = 2.00 m the fringes are 5.2 mm apart for 650 nm. The 600 lines per mm grating (d = 1.667 micrometres) puts first-order spots at 22.95 degrees (0.847 m from the centre on a 2.00 m screen) and second-order at 51.3 degrees (0.623 m from the centre on a screen 0.500 m away); no third order exists because 3 lambda exceeds d. The learner knows it worked when the measured wavelength lands within about 2 per cent of the laser's nominal value.

What changes

What you change: order number m (and slit separation)
What you measure: angle to the bright fringe
What you keep the same: wavelength
screen distance
grating perpendicular to the beam

Common misconceptions

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

Fringes get closer with a narrower slit separation; they get farther apart.
Higher orders are brighter; they are fainter and eventually cannot form.

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 PH11/12-3 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-12-03 PY-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities