Physics 11–12 · Year 12
Speed of light from the standing waves in a microwave oven (syllabus practical)
Module 7: The Nature of Light (Electromagnetic Spectrum)
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The idea
Microwaves are light of long wavelength; inside an oven they form standing waves whose hot spots sit half a wavelength apart, so the spacing and the oven's stated frequency give the speed of light.
Safety card
Hazards
- hot melted chocolate or marshmallow
- arcing if foil or metal goes in the oven
- running an oven with a damaged door or seal
Controls
- remove all foil, heat in short bursts and let the plate cool before handling
- use an oven with an intact door and seal
- do not eat anything prepared in the laboratory
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/).
What you need
- An undamaged domestic microwave oven with its rating plate showing the operating frequency (2450 MHz on most ovens), turntable removed
- A flat bar of chocolate with the foil removed, or a microwave-safe plate covered with mini marshmallows
- Ruler graduated in millimetres, toothpicks to mark the melted spots
How to do it
- Read the operating frequency from the oven's rating plate.
- Remove the turntable (or stop it turning), place the chocolate or marshmallows flat in the centre, and heat for about 15 seconds, only until the first spots soften.
- Mark the centres of the melted or puffed spots with toothpicks and measure the distance between neighbouring spots along one line; measure three pairs and average.
- The spacing is half the wavelength: compute lambda = 2 x spacing and c = f x lambda.
- Repeat with a fresh bar at a different position and compare.
What you should see
At 2450 MHz the wavelength is 0.1224 m, so neighbouring hot spots should be 61.2 mm apart. A measured spacing of 60 mm gives c = 2 x 0.060 x 2.45e9 = 2.94e8 m/s, 1.9 per cent below 2.998e8 m/s. Spot centres are hard to judge closer than a few millimetres, so a result within about 5 per cent is good, and pooling the class's spacings narrows it.
What changes
- What you change
- position of the sample in the oven
- What you measure
- spacing of the melted spots
- What you keep the same
- oven frequency
- heating time
- sample thickness
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Microwaves are not light; they are the same electromagnetic wave at a longer wavelength and travel at the same speed.
- Neighbouring hot spots are one wavelength apart; they are half a wavelength apart.
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 2027PH12-14PH11/12-3
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-03PY-12WS-03
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
- education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-7-guide.docx
- www.open.edu/openlearn/science-maths-technology/science/physics-and-astronomy/physics/measure-the-speed-light-your-microwave
- www.lsop.colostate.edu/wp-content/uploads/sites/6/2021/02/measuring-the-speed-of-light.pdf
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes