Melt the chocolate, measure the light.

Sometimes called the "chocolate speed of light experiment" (marshmallows and cheese work too). A spinning turntable smears the standing-wave pattern evenly across a whole rotation, which is exactly why ovens have one — to heat food evenly, the opposite of what this experiment wants — so the first step is always to remove it. With the turntable out, running the oven briefly on a flat tray reveals hot spots. In the ideal single-mode picture used here, adjacent antinodes are spaced half a wavelength (λ/2) apart; real oven cavities can support overlapping modes, making the melted pattern less regular. A household microwave's magnetron runs at a fixed, nameplate-printed frequency of f = 2.45 GHz, so λ = c/f ≈ 12.24 cm and λ/2 ≈ 6.12 cm. The frequency slider changes the diagram's theoretical geometry; it does not make other equipment suitable for the test. Wi-Fi routers, phones, mobile-network gear and ordinary radio transmitters do not confine or deliver enough power to melt chocolate. The equation requires compatible SI units: frequency is converted to hertz and spacing to metres before multiplying. If GHz and cm are multiplied directly, 1 GHz·cm = 10⁷ m/s. For example, 2 × 2.45 × 6.39 = 31.311 therefore means 31.311 × 10⁷ m/s = 3.1311 × 10⁸ m/s. A ruler reading off by a couple of millimetres, magnetron-frequency tolerance, overlapping modes, and blurred melted spots all add real error. Read more on Wikipedia's microwave oven article.