Over 11s # Measure the speed of light using chocolate

#### Nuke your chocolate and measure a fundamental physics constant

Remember E=mc2, Einstein's famous equation? The 'c' stands for the speed of light. You don't need fancy equipment to measure it. All you need is a microwave, ruler, bar of chocolate and a calculator.

How to:

1. Take the turntable out of the microwave. You need the chocolate to stay still whilst you heat it.
2. Put a plate upside down over the thing that rotates the turntable (does that have a name? For now I'm going with 'rotator').
3. Put your chocolate in the middle of the plate.
4. Heat the chocolate until it starts to melt in two or three places. This should take about 20 seconds.
5. Take the chocolate out of the microwave - carefully! It will be hot. Measure the distance between the melted spots.
6. If your microwave is a standard model, it will have a frequency of 2.45 gigahertz. This means that the microwaves move up and down 2.45 billion times per second. Check in your microwave manual if you're not sure of the frequency.
7. Multiply the distance between the spots on the chocolate bar by two. Multiply that by 2,450,000,000 (2.45 gigahertz expressed as hertz).

Distance between two melted spots of chocolate x 2 x 2450000000 = z

What answer do you get for z? The speed of light is 299,792,458 metres per second.

Remember, if you measured the distance between the melted spots in centimetres, z will be in centimetres per second. To get an answer in metres per second, divide z by 100.

What's happening?

Microwaves are a type of electromagnetic radiation, just like light waves. Microwaves also travel at the speed of light. If you measure how fast they are travelling, you should get a result close to the speed of light.

Wavelength

When you measure the distance between two melted spots you can work out the wavelength of the microwaves.

Measuring the distance between melted spots gave you half a wavelength. You need to multiply the distance by two to get a whole wavelength. The distance between two melted spots is half a wavelength

Frequency

Now you know the wavelength you need to know the wave frequency. Wave frequency is how many times a wave bounces up and down in one second. This is 2.45 gigahertz in most microwaves. This means that a wave will move up and down 2.45 billion times per second.

Speed

Speed of light = wavelength x frequency

The distance between each melted spot should be around 6 centimetres.

6 x 2 x 2450000000 = 29400000000 cm/s

This is equivalent to 294,000,000 metres per second. That's pretty close to the speed of light!

Now you've satisfied your curiosity, you can eat the chocolate. A well deserved reward for you hard work.

This experiment featured on the Null Hypothesis and Wired.

### Curriculum information

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