Microwaves are radio waves at the high-frequency end of the radio spectrum, with wavelengths from about 1 millimeter to 1 meter (3.3 feet). The explorer shows a microwave with a wavelength of 1 centimeter (0.39 inch). That is 10 times the shortest microwave and one-hundredth of the longest, toward the short end of the band.

Wavelength

Microwaves, like light, are electromagnetic waves: waves of electric and magnetic energy moving together through space. A wave's wavelength is the distance from one crest to the next. Its frequency is the number of crests that pass a point each second, counted in hertz. The shorter the wavelength, the higher the frequency.

A 1-centimeter microwave has a frequency of about 30 gigahertz, so about 30 billion crests pass a point every second. The Occupational Safety and Health Administration (OSHA), the US workplace-safety agency, defines microwaves as frequencies from 300 megahertz (300 million crests a second) to 300 gigahertz. Those frequencies match wavelengths from 1 meter down to 1 millimeter.

The International Telecommunication Union (ITU), which coordinates radio use worldwide, names radio bands by the length of their waves. It calls 3–30 gigahertz "centimetric waves" and 30–300 gigahertz "millimetric waves". At 29.98 gigahertz, a 1-centimeter wave sits at the very top of the centimetric band.

Size comparisons

Two 1-centimeter waves laid end to end would span a little more than the width of a U.S. penny, which is 19 millimeters (0.75 inch) across.
A standard glass marble, 1.6 centimeters (5/8 inch) across, is about 1.6 wavelengths wide.
Microwave ovens use waves about 12 centimeters (4.7 inches) long, 12 times the length of this one.
The longest waves of visible light, red light, are about 700 nanometers long. A nanometer is a billionth of a meter, so a 1-centimeter microwave is about 14,000 times as long.

What microwaves are used for

A microwave oven makes its waves with an electron tube called a magnetron. The waves bounce off the oven's metal walls and pass through glass, paper and plastic, but food absorbs them. They make the water molecules in the food vibrate, and that motion produces the heat that cooks it.

Microwaves also carry telephone and television signals, and radar uses them to detect speeding cars. Doppler weather radar uses them to follow storms, and most communication satellites use them to send signals to the ground. In space communications, frequencies of 27.5–30 gigahertz, with waves 1.0–1.1 centimeters long, are known as the 30-gigahertz band.

Microwaves from the early universe

In 1965, Arno Penzias and Robert Wilson of Bell Telephone Laboratories were measuring radio noise with a 20-foot (6-meter) horn antenna in Holmdel, New Jersey. At a wavelength of 7.35 centimeters, about seven times the explorer's wave, they picked up more noise than they could account for. It came equally from every direction in the sky.

A companion paper by physicists at Princeton University offered an explanation. The noise was the cosmic microwave background: light released when the universe was about 380,000 years old, which fills the whole universe. Penzias and Wilson shared the 1978 Nobel Prize in Physics "for their discovery of cosmic microwave background radiation".

Sources

Microwaves. NASA Science, 2023.
Anatomy of an Electromagnetic Wave. NASA Science, 2023.
What are the Energy Range Definitions for the Various Types of Electromagnetic Radiation?. NASA High Energy Astrophysics Science Archive Research Center, 2021.
Visible Light. NASA Science, 2023.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Radiofrequency and Microwave Radiation. Occupational Safety and Health Administration.
Recommendation ITU-R V.431-8: Nomenclature of the frequency and wavelength bands used in telecommunications. International Telecommunication Union, 2015.
Microwave Oven Radiation. US Food and Drug Administration, 2023.
Coin Specifications. United States Mint, 2026.
Tournament Rules. National Marbles Tournament.
A Measurement of Excess Antenna Temperature at 4080 Mc/s. Penzias and Wilson, The Astrophysical Journal, 1965.
The Nobel Prize in Physics 1978. Nobel Prize Outreach.

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