The Speed Of Light From Your Microwave

Age

Elementary School, but better for Middle School, High School

Format

Hands-on

Materials

   Microwave
   Marshmallows (1/2 bag)
   8x8 baking dish
   Ruler
   Calculator
   Electromagnetic spectrum poster
   Wave/speed of light poster 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

Don’t eat the marshmallows.

Preparation

Remove the revolving plate from the microwave. Note the frequency of the microwave from a label on the back.

Demonstration

Place the marshmallows in the baking dish so that they stand side by side and fill the dish. Place the dish inside the microwave, butted up against a corner. Microwave the marshmallows for 1-2 minutes, until a standing wave in the marshmallows can be seen. Remove the dish, measure the wavelength of the standing wave. Multiply the wavelength of the wave (in meters) by the frequency on the back of the microwave (in hertz (1/s)), this will give you the speed of light. The actual speed of light is 3 x 108 m/s, you should get something on the right order of magnitude.

What to Say

Before starting, explain to the audience what a wave is, including what a wavelength is and how it can be measured. Start by asking the kids where they have seen a wave (the ocean). In the ocean, only the positive portion of the wave is seen. You will most likely have to measure a half a wavelength, so explain that each portion of the wave is equal, and you can just multiply the half a wavelength by two to get the entire wavelength although it is not visible in the marshmallows.

Next, explain where microwaves lie in the electromagnetic spectrum. Visible light is only a small portion of the electromagnetic spectrum, and the wavelengths of visible light are extremely small compared to the wavelengths of microwaves.

Light travels with a speed called the speed of light. This is 3 x 108 m/s or approximately 186,282.397 miles per second (mention this number). In order to calculate the speed of light from the microwave, the wavelength of the microwaves from the appliance must be measured and multiplied by the frequency on the back of the appliance. The frequency is how many microwaves are emitted per second, and has units of ‘per second.’

The microwaves in the appliance will affect the marshmallows according to where in the wave the marshmallows lie. This will produce a ‘wave’ in the marshmallows which is the same size as the microwave.

Why It Is

The speed of light is 3 x 108 m/s, that is, light travels 300,000,000 meters in one second. Light is a wave, and travels as a wave. By measuring the length of the microwaves in the appliance and using the frequency from the manufacturer, the speed of light can be approximated from the standing wave in the marshmallows.

Real Life Examples

Waves in the ocean. Also, you can mention that since the light from lightning travels so fast, it takes some time for the thunder to catch up (~5 sec/mile).