When you drop a stone into a pool of water, you see waves. Ripples in the water bounce up and down. The waves form circles around the spot where the stone hit the water. The ripples start small but then they move away from the center, in bigger and bigger circles. The frequency of the waves represents how quickly they are bouncing up and down.
Microwaves are a type of radio wave. Appliances such as your radio, cordless telephones, cell phones, and television all function by radio waves. Radio waves are like water waves, but you can't see them.
In addition, radio waves work on a much smaller scale. Everything in the universe is made up of atoms (the fundamental building blocks of all matter). The most mobile parts of an atom are called electrons. When radio waves hit an object, they make the electrons in that object bounce around.
High frequency radio waves have more energy than low frequency waves do.
Microwaves are very high-frequency radio waves. They are used in cell phones, wireless Internet, and in microwave ovens.
The waves in cell phones and wireless Internet do not get very much electricity, so these waves are very weak. In contrast, lots of electricity runs through a microwave oven, so microwaves are strong.
Just as water waves make things move, microwaves make atoms move. The atoms bump into each other, and the resulting friction makes the food get hot.
In a microwave oven, a radio makes microwaves and sends them in one direction. They are aimed at a spinning fan that sits above or beside the food inside. Sometimes you can see the fan, but most of the time it is hidden behind plastic.
When the microwaves hit the spinning fan, the waves bounce off and hit the food. The microwaves then get absorbed by the fats, sugars, and especially water in the food. Once absorbed, the microwaves cause the electrons in the food to vibrate. This generates heat, which can then evenly heat up your food.
Microwaves can bounce around inside the oven. The metal walls of the oven keep the microwaves from escaping into the surroundings. Even though you can see the food while it's cooking, the microwaves won't bounce out of the glass door because the metal screen stops them. Still, it is not good to be too close to the oven when it is cooking.
For more fun information about the science of microwave ovens, you should explore these interactive websites!
- Dr. Electric's Microwave Oven Laboratory at http://www.discovery.panasonic.co.jp/en/lab/lab02mw/
- Professor Lee's Microwave Oven Laboratory at http://www.colorado.edu/physics/2000/microwaves/
Get two crackers that are the same size and type. Moisten one cracker with room-temperature water. Put both crackers onto a napkin and microwave them for 7 seconds. Safety note: wait 20 more seconds before opening the microwave door so that the heat will have a chance to spread out evenly over each cracker! Feel both crackers with your finger. Which one is warmer? Why?