In many households, microwaves play a big part in heating and cooking foods. Microwaves cook food by causing water molecules in it to vibrate and absorb energy. This energy heats up the food without burning it or making it radioactive. Microwaves can also be used to thaw or defrost frozen food in a fraction of the time it takes to do so with other methods.
As the name suggests, microwaves are part of the electromagnetic spectrum that includes gamma rays, X-rays, visible light, infrared and radio waves. Their wavelengths are between 300 MHz (0.1 cm) and 1 meter, which puts them in the middle of the spectrum, between the infrared window and the radio wave frequency range.
While radio waves can travel a great distance through air, microwaves do not travel far above the Earth’s surface. This is because the atmosphere starts to absorb microwaves at a certain frequency, as shown in the graph below. At higher frequencies, the microwaves are not absorbed as much.
Various devices can generate microwaves. Vacuum-tube devices like the klystron and magnetron are used in higher power applications, such as radar systems and microwave ovens. Solid-state devices like the Gunn diode and tunnel diode can also produce microwaves.
Exposure to high levels of microwave radiation can cause burns, especially in areas such as the eyes and testes where there is not much blood flow to carry away excess heat. The lens of the eye is particularly sensitive to microwaves and prolonged exposure can lead to cataracts.