Measuring the Speed of Light

It's taken a bit of coordinated effort to do it, but the next video episode of Conversations in Science: Experiments with Kids is now available on the KLRNRadio YouTube channel.

This time, I was joined by my daughter in a fun experiment on how we can measure the speed of light using our microwave.

The Experiment

You read that right, folks. You can measure the speed of light using your standard microwave.

For this experiment, you'll need the following:

  • Microwave
  • Bowl and plate
  • Calculator
  • Measuring tape or ruler

And the most important item of all:

CHOCOLATE

(Oh, yum, yum, yum! I love it when science gets tasty.)

Step 1

Disable the turntable within your microwave by removing the turntable plate, then place a microwaveable bowl upside down over the mechanism. Place a microwaveable plate on top of the upside down bowl.

Step 2

Place a bar of chocolate that measures approximately 8 cm by 8 cm onto the plate in the center of the microwave. Microwave on high for approximately 30 seconds. (Times may vary depending on the wattage of the microwave.)

Step 3

Measure the distance between two neighboring melted sections. Be advised that it can be difficult to find the melted sections. You may have to perform this experiment repeatedly just to get a good measure. (Yum, yum! More chocolate.)

Step 4

Convert your measurement into meters and multiply by 2, then multiply by the frequency of your microwave in Hertz. You can find the frequency of your microwave on the back. Most domestic microwaves use a frequency of 2450 MHz (2,450,000,000 Hz).

If you have performed your calculations correctly, then you should have an answer that is approximately 300,000,000 m/s.

The Science

When people think of light, they tend to think of what we can see, but visible light is only a small narrow band in the electromagnetic spectrum. The full spectrum includes infrared, ultraviolet, X-rays, gamma rays, microwaves, and radio waves.

Electromagnetic Spectrum. The spectrum of waves includes infrared rays, visible light, ultraviolet rays, and X-rays. Human eyes are only sensitive to the range that is between wavelength 380 nanometers and 780 nanometers in length.

Almost everything on our planet will emit light in some form. We humans emit infrared light in the form of a heat source. (BTW, this is the principle behind night vision technology.)

When discussing light, physicists have a variety of models at their disposal, one of which is that light is a wave. In the case of microwaves, that is exactly what we're dealing with, hence, the term "microwave".

For a wave that travels, a point will move through a full range of sinusoidal motions. (Image source: Wikipedia)

A standing wave is where certain points of the wave are in a fixed location (known as nodes) and everything else is moving around these points (anti-nodal regions). (Image source: Wikipedia)

Waves can be described as traveling waves, rippling along. One example is when you throw a pebble into a still pool of water. The waves travel outward from the origin along the surface of the water.

Waves can also be described as standing waves. These waves are where the end points (and sometimes points in the middle) are fixed, known as nodes, and everything else is moving about, known as anti-nodal regions.

The waves generated in a microwave are standing waves. The heating of food occurs in the anti-nodal regions. (Source: The Naked Scientist)

The waves a microwave generates are standing waves with heating of the food occurring in the anti-nodal regions. A turntable is used to ensure that the anti-nodes can heat the food evenly.

In this experiment, you are measuring the distance between two consecutive anti-nodes. This measurement will give you half the wavelength ( λ ). If you multiply λ by the frequency of the wave ( f ), then you have the speed of the wave.

Light, all forms of light, travels at a constant speed ( c ) through air or a vacuum. This is accepted to be be 3 x 108 m/s (or 300,000,000 m/s). Given the relationship of c = λf, then for a frequency of 2450 MHz (or 2,450,000,000 Hz), you can expect a wavelength of approximately 0.12 m (12 cm). Half of that is 6 cm, which is what we measured in our experiment.

How close did get with your measurements?

For more information about the microwave speed of light experiment and to see it in action, check out the special video edition of Conversations in Science: Experiments with Kids on the KLRNRadio YouTube channel. (Video link is also shared below.)

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© Copyright, Judy L Mohr 2017

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