Quick answer: The electromagnetic spectrum is a family of transverse waves that travel at 3.0 × 10⁸ m/s in a vacuum. This Physics 9-10 lab on electromagnetic spectrum gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can a microwave oven and a chocolate bar help you measure the speed of light? In this lab the pattern of melted spots reveals the wavelength of the microwaves, and the frequency on the oven label gives you the speed with c = f × λ.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS4-4, HS-PS4-5
Aim
To measure the speed of light by finding the wavelength of microwaves in a microwave oven and using the frequency printed on its label.
Equipment
- Microwave oven (the frequency is printed on the back or inside the door, usually 2.45 GHz)
- Flat microwave-safe plate with the turntable removed
- Bar of chocolate or a slice of cheese spread flat
- Ruler in centimetres
- Oven gloves
- Pencil and notebook
Safety
Never run a microwave oven empty. Heat for 15-20 seconds at a time, use oven gloves because the plate and melted food are hot, and do not eat the food afterwards. Ask an adult to supervise.
Method
- Take out the rotating turntable so the food stays still, and note the frequency printed on the oven.
- Place the chocolate bar on the flat plate, on a fixed spot.
- Heat for 20 seconds, then stop and look. Repeat in 10-second bursts until several melted spots appear.
- Measure from the centre of the first melted spot to the centre of the last visible one in a row, and count the gaps between spots.
- Calculate the mean spacing d between neighbouring spots, then double it to get the wavelength.
- Repeat with a fresh piece of chocolate for each trial.
Results
Sample measurements (oven frequency 2.45 GHz):
| Trial | Number of gaps | Distance first to last spot (cm) | Spacing d (cm) |
|---|---|---|---|
| 1 | 4 | 24.0 | 6.00 |
| 2 | 3 | 18.6 | 6.20 |
| 3 | 4 | 24.6 | 6.15 |
| 4 | 3 | 18.3 | 6.10 |
Analysis questions
Question 1 Mean spacing
Calculate the mean spacing d from the table to 3 significant figures.Question 2 Wavelength
Explain why the wavelength is twice the spacing between melted spots, then calculate it.Question 3 Speed of light
Use c = f × λ to calculate the speed of light from your results.Question 4 Percentage difference
Compare your value with 3.00 × 10⁸ m/s and calculate the percentage difference.Question 5 Turntable
Explain why the turntable must be removed.Question 6 Uncertainty
Estimate the range of the spacings and say which source of error is largest.Question 7 Improvement
Suggest two improvements to give a more reliable value.
