Microwave ovens produce a type of energy that can travel through the air, called radiation. Light that comes out of light bulbs is also a type of radiation, but we can see this visible light radiation with our eyes. On the other hand, our eyes cannot detect microwaves, so we cannot see the microwave radiation shining down on our food while a microwave oven is running. The best way to think about a microwave oven is as a machine that acts like a "light bulb" that produces microwave radiation and shines that microwave radiation down onto your food to heat it up.

The reason that microwave radiation is much better at heating food than visible light is difficult to explain. A simplified explanation is that microwave radiation is very good at transferring energy (heat) to water. Since most foods contain a lot of water, this results in the transfer of a lot of heat into your food. For some very complex reasons, other types of radiation, like visible light, are much worse at transferring energy into water. As a result, we cannot just shine a normal light bulb on food to heat it up quickly.

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Microwave ovens work by using 2.45 GHz frequency electromagnetic waves know as microwaves to heat the water in food. Water absorbs the energy from the electromagnetic waves and turns that energy into thermal motions causing the temperature of the food to increase.

Side note: microwaves are electromagnetic waves and as such are part of the electromagnetic spectrum. Other examples of electromagnetic waves include radio waves, infrared light, visible light, ultra violet light, x-rays, and gamma rays. The difference between all of these electromagnetic waves are their wavelength, frequency, and energy per photon.

Microwave ovens transmit a high power electromagnetic wave into your food. It's the same type of wave used for wifi internet connections but at a much higher power level. The microwave oven is shielded so that all of this wave stays inside the microwave oven. The wave, like all electromagnetic waves, consists of a rapidly changing electric and magnetic field. Most materials will absorb some energy from radio waves, but anything containing water will absorb a lot of energy because water is a polar molecule so the molecules are easily jostled around by the rapidly changing electric field. Heat is just the random microscope motion and vibration of atoms and molecules in a material, so when it is jostled around by the wave it heats up.

Microwave is a color of light, too low-energy for you to see, lower-energy even than infrared, but higher-energy than radio.

Microwave ovens produce large amounts of this light. Because of the size of the light waves (micrometers, hence the term microwave), they are capable of spinning water molecules as they go past. This spinning of the water molecules then rubs off on all of the other molecules they are with, causing the entire substance to heat up.

A microwave oven uses microwave radiation to heat water. A microwave oven produces microwave radiation with a frequency of about 2.45 GHz, which is similar to a cordless phone or your wifi router. However, the radiation is much more intense, so when this large amount of energy is transferred to water molecules in your food, it causes the food to heat up. Specifically, the microwave radiation causes water molecules to move around, and when they bump into other molecules, they transfer this energy in the form of heat.