Microwave Oven Hotspots: Lesson for Kids

Instructor: Nick Rogers
Learn about why some places in your microwave oven are hotter or colder, and use the definition of frequency and wavelength to predict their locations! Perform an experiment to measure the speed of light in your kitchen.

A Simple Idea - and Food in Seconds!

Microwaves are a form of radiation that can penetrate water, fat and sugar molecules. Inside your microwave oven, an object called a magnetron creates microwaves, which bounce around inside because they can't pass through metal. As the microwaves move through your food, the molecules become excited and start to move. The friction this creates results in heat. Voila - you have hot food in seconds.

Microwaves are a type of wave, and have a frequency and wavelength. Waves have troughs and valleys - high points and low points. As a result of this, microwaves tend to have hot spots and cold spots. Can you guess where the hot spots would be?

Close-up view of a wave

If you guessed that the hot points are at the peaks/troughs, and the cold points are at zero, you were correct!

The wavelength ( λ ) is how long it takes for the wave to complete one cycle and return to its starting point. The frequency (f) of this wave measures how fast it completes each wavelength. Frequency is usually measured in cycles per second, or hertz (Hz). We can relate the speed (V) of the wave to its frequency and wavelength through multiplication:

V = λ * f

The velocity of a microwave is equal to the speed of light, or 299,800,000 meters/second. It makes sense that microwaves would also have very high frequencies. You can find the frequency of the microwaves that your microwave oven uses by checking for a label inside it. Mine operates at 2450 MHz. The 'MHz' is common notation for Megahertz, or million hertz. This means that the microwaves inside my microwaves complete 2.45 billion (2,450,000,000) cycles each second!

Why is Some Food Cold?

When you cook food inside your microwave, the energy is not applied evenly to the food. The peaks and troughs in the waveform create areas that are hotter and areas that are colder. From the picture above, we know that hot spots happen at adjacent peaks. Two hot spots close together should be a distance apart equal to half of the wavelength of the microwave.

You can determine the particular wavelength of your microwave from the formula that we have above. Since we know that microwaves travel at the speed of light and we also know the frequency, we can compute λ:

λ = V / f = 299,800,000 / 2,450,000,000 = 0.122 m

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