The speed of light can be measured with a microwave oven and a chocolate bar. The oven's standing waves melt the chocolate in a pattern that reveals the wavelength of the radiation. A simple calculation converts that wavelength into a speed, and the result comes close to the accepted value of 299,792,458 meters per second. The experiment requires only a microwave, a chocolate bar, a ruler, and the frequency printed on the back of the oven.
A microwave oven heats food through standing waves. The magnetron generates microwave radiation that reflects off the oven's metal walls and interferes with itself. The forward and reflected waves combine to create a fixed pattern of peaks and troughs called standing waves. At the antinodes, the wave amplitude is greatest, and the electric field is strongest; at the nodes, the amplitude is zero. Because the electric field is strongest at the antinodes, the water and fat molecules in the chocolate rotate most vigorously there and generate heat, melting the chocolate. At the nodes, the field is zero, so no heat is generated. This is why microwaves heat unevenly: the hot spots are the antinodes, and the cold spots are the nodes. A turntable rotates food through this pattern to average out the hot and cold regions, which is why most ovens include one. The melted spots on a chocolate bar mark the antinodes directly. Adjacent antinodes are half a wavelength apart, so the measured spacing is λ/2. For a 2.45 GHz oven, the full wavelength is about 12.2 centimeters, so the half-wavelength you measure should be roughly 6.1 centimeters.
To run the experiment, remove the turntable from the microwave and place a chocolate bar on a microwave-safe plate. Removing the turntable stops the chocolate from moving through the standing wave pattern, so the melted spots stay fixed. Never run the oven empty; without food to absorb the radiation, the magnetron can be damaged. Heat the chocolate in short bursts of 20 to 30 seconds until spots just begin to melt, then remove it immediately. Overheating melts the entire bar and erases the pattern. You want a few distinct melted spots, not a uniformly soft bar. Make sure the chocolate is at room temperature before starting.
Measure the distance between the centers of adjacent melted spots with a ruler. That distance is half the wavelength of the microwave radiation. Use a flat bar without nuts or other inclusions, as uneven surfaces make the melted spots harder to read. If the spots are irregular or the bar melts unevenly, try a flatter bar or a shorter heating time. A bar with a uniform surface gives the cleanest pattern. Take several measurements across the bar and average them to reduce error.
Find the microwave's frequency on the label on the back of the oven. Most household microwaves operate at 2.45 GHz, but check your own oven's label rather than assuming. Write the frequency down before you start, because you will need it for the calculation. If your oven's frequency differs from 2.45 GHz, use the label value instead.
Multiply the measured half-wavelength by two to get the full wavelength. Then apply the wave equation: speed = frequency × wavelength. This equation holds for any wave, from sound to light. The calculation is straightforward: multiply the frequency in hertz by the wavelength in meters.
A typical measurement produces a half-wavelength of about 0.061 meters. Doubling that gives a full wavelength of about 0.122 meters. With a frequency of 2.45 GHz, the calculation gives: 0.122 meters × 2,450,000,000 Hz = 299,000,000 meters per second. The accepted speed of light is 299,792,458 meters per second. The kitchen estimate comes close to that value, usually within a few percent. The main source of error is the measurement of the melted spot spacing, which can be off by a few millimeters. A typical measurement error of a few millimeters in the spot spacing leads to a few percent error in the final speed.
