Measuring the Speed of Light with a Microwave

The speed of light is 299,792,458 m/s and great pains were taken to measure it as accurately as possible through various methods.  That's also an extremely fast speed, a speed that nothing with mass can reach or exceed!  Despite the complex experiments designed to measure the speed of light, you can come close to measuring the speed of light on your own using simple items likely found in your kitchen.

To measure the speed of light, all you need is a working microwave and some larger food items (marshmellows, chocolate bars, egg yolk, etc.).  This works because a microwave works by bombarding your food with light at microwave wavelengths.  We typically think of light as what we see with our eyes, but most of the light in the Universe is invisible to our eyes.  Body heat, for example, is light emitted at infrared wavelengths.

Most microwaves operate at 2450 MHz.  That's 2,450 million hertz.  You can double check this by looking somewhere on the inside door of your microwave.  There's probably a sticker or tag of some kind displaying the operating frequency.  The speed of light is equal to the frequency of the wave emitted times the wavelength of the light emitted.

Speed of light = frequency * wavelength

Since we know the frequency, we just need to determine the wavelength to find the speed of light.  The wavelength can be found by noticing hot spots in your food as it cooks.  Most microwaves today have a turntable that slowly turns your plate of food.  This allows for more even cooking.  Without the turn table, the waves will produce maximum hot spots at various locations in your food.  These hot spots are located equal distances apart, a distance that is exactly 1/2 the wavelength.  Therefore, if you can measure the distance between these hot spots, you can find the wavelength and thus find the speed of light.

We first started with a chocolate bar.

This didn't work so well.  The chocolate melted from the bottom and it was hard to pinpoint exact hot spots.  The plate is sitting on a bowl covering the turning apparatus.  Next we tried marshmellows.

Again, this didn't work very well.  The marshmellows expanded as expected, but made it difficult to measure hot spots.  Next we scrambled up an egg and tried that.  Success!

Notice the two spots where the egg yolk has started to harden.  These are hot spots where waves are constructively interfering.  The distance between these hot spots is equal to 1/2 the wavelength.

Measuring from the center of one hot spot to the center of the other hot spot, we measured a distance of 2 5/8 inches (6.6675 cm).

v = (0.06675*2)*(2,450,000,000)

v = 3.26 x 10^8 m/s

Comparing this to the expected value of 3 x 10^8 m/s, we were off by 8.7%.  Not too shabby given our measuring tools were a microwave and a tape measure!