Microwave facts for kids

Kids Encyclopedia Facts

This page is about the electromagnetic wave. For the cooking appliance, see Microwave oven. For other uses, see Microwaves (disambiguation).

Microwaves are a type of electromagnetic radiation, which is a form of energy that travels through space. Think of them as tiny radio waves, but even shorter! They are part of the same family as visible light, X-rays, and infrared waves. Their wavelengths are usually between one meter and one millimeter. This means their frequencies are very high, ranging from 300 MHz to 300 GHz. The name "micro-wave" comes from the idea that they are "small" compared to the longer radio waves used before.

A tall tower with many dish antennas for sending and receiving microwave signals. These are used for communication over long distances.

Microwaves travel in straight lines, much like light. They don't bend around hills or bounce off the Earth's atmosphere like some other radio waves. Because of this, microwave communication links on Earth are usually limited to about 40 miles (64 km). These amazing waves are used in many parts of our daily lives. You might know them from microwave ovens that cook food quickly. But they also power wireless networks like Wi-Fi, radar systems, satellite communication, and even help in medical treatments and industrial heating.

Understanding the Electromagnetic Spectrum

Microwaves fit into the larger electromagnetic spectrum. This spectrum includes all types of light and energy waves. Microwaves are found between regular radio waves and infrared light. Here's a simple look at where microwaves are in the spectrum:

Electromagnetic spectrum
Name	Wavelength	Frequency (Hz)
Gamma ray	< 0.01 nm	> 30 EHz
X-ray	0.01 nm – 10 nm	30 EHz – 30 PHz
Ultraviolet	10 nm – 400 nm	30 PHz – 750 THz
Visible light	400 nm – 750 nm	750 THz – 400 THz
Infrared	750 nm – 1 mm	400 THz – 300 GHz
Microwave	1 mm – 1 m	300 GHz – 300 MHz
Radio	≥ 1 m	≤ 300 MHz

Sometimes, microwaves are seen as a type of radio wave. Other times, they are considered a separate kind of radiation. It's just a way of organizing them.

Microwave Frequency Bands

Different parts of the microwave spectrum are given letter names. These names help scientists and engineers talk about specific frequency ranges. The system started during World War II for secret radar projects. Here are some common microwave frequency bands and their uses:

Microwave Frequency Bands
Designation	Frequency Range	Wavelength Range	Common Uses
L band	1 to 2 GHz	15 cm to 30 cm	GPS, mobile phones, amateur radio
S band	2 to 4 GHz	7.5 cm to 15 cm	Weather radar, microwave ovens, Wi-Fi, Bluetooth, GPS
C band	4 to 8 GHz	3.75 cm to 7.5 cm	Long-distance communication, Wi-Fi
X band	8 to 12 GHz	25 mm to 37.5 mm	Satellite communication, radar, space communication
K_u band	12 to 18 GHz	16.7 mm to 25 mm	Satellite communication
K_a band	26.5 to 40 GHz	5.0 mm to 11.3 mm	Satellite communication

These bands are used for many different technologies we rely on every day.

How Microwaves Travel

Microwaves travel in straight lines, just like light. This is called line-of-sight propagation. Unlike some lower frequency radio waves, they don't follow the curve of the Earth or bounce off the ionosphere (a layer in our atmosphere).

Atmospheric Microwave Transmittance at Mauna Kea (simulated)

This graph shows how microwaves are absorbed by the atmosphere. The dips mean more absorption.

This means that for microwaves to communicate, the transmitting and receiving antennas usually need to "see" each other. On Earth, this limits communication links to about 30 to 40 miles (48 to 64 km). Microwaves can also be absorbed by moisture in the air, like rain. At very high frequencies, gases in the atmosphere can also absorb them. This limits how far they can travel, sometimes to just a few kilometers.

Tropospheric Scatter

Sometimes, a small part of a microwave beam can scatter when it passes through the lower atmosphere, called the troposphere. Even if a receiver is beyond the visual horizon, it can pick up these scattered signals. This method, called tropospheric scatter, allows communication over distances up to 300 km.

Microwave Antennas

The short wavelengths of microwaves are very useful for antennas.

Special metal pipes called waveguides are used to carry microwaves. This image shows waveguides and a diplexer in an air traffic control radar system.

For portable devices like cell phones and Wi-Fi devices, antennas can be made very small, just a few centimeters long. This is why your phone can fit in your pocket! For longer distances or specific tasks like radar, microwaves can be focused into narrow beams. This is done using antennas like dish antennas, which can be from half a meter to several meters wide. Narrow beams are great because they don't interfere with other equipment nearby. Instead of regular wires, microwaves are often carried by special metal pipes called waveguides. This is because regular wires lose too much power at microwave frequencies.

How Microwaves are Made

Microwaves can be generated in different ways.

Inside a cavity magnetron, used in microwave ovens (left). A microstrip circuit for higher frequencies (right).

For high-power uses, special vacuum tubes are used. The most famous is the magnetron, which you'll find in microwave ovens. Other tubes like klystrons and traveling-wave tubes (TWTs) are also used. These devices work by controlling the movement of electrons in a vacuum. For lower power uses, solid-state devices are common. These include Gunn diodes and IMPATT diodes. These small electronic parts are found in many modern gadgets.

Inside a radar speed gun. The grey part attached to the copper horn antenna is a Gunn diode, which creates the microwaves.

All warm objects naturally give off a small amount of microwave radiation. Scientists use special tools called microwave radiometers to measure this. Even space objects like the Sun and distant galaxies emit microwaves. Radio telescopes study these signals to learn about the Universe. For example, the cosmic microwave background radiation (CMBR) is a faint microwave glow from the Big Bang.

Everyday Uses of Microwaves

Microwaves are essential for many modern technologies. They are great for sending information from one point to another because they can carry a lot of data quickly. Also, their antennas can be made smaller.

Communication with Microwaves

A satellite dish on a house. It receives satellite television signals from a satellite orbiting 22,000 miles (35,700 km) above Earth.

Before fiber-optic cables, most long-distance phone calls traveled through networks of microwave radio relay links. These systems could carry thousands of phone calls at once. Today, wireless networks like Bluetooth and Wi-Fi use microwaves in the 2.4 GHz and 5 GHz ranges. Your mobile phone also uses microwaves (around 1.8 and 1.9 GHz) to connect to cell towers. Satellite communication systems use microwaves to send TV, phone, and internet signals around the world. Satellite TV dishes on homes receive microwave signals from satellites in space.

Navigation Systems

Global Navigation Satellite Systems (GNSS), like the American Global Positioning System (GPS), use microwaves. These systems broadcast signals in the 1.2 GHz to 1.6 GHz range. Your GPS device receives these signals to figure out your exact location on Earth.

Radar Technology

The large dish antenna of an airport surveillance radar. It sends out microwave beams to find aircraft around an airport.

Radar uses microwaves to find objects and measure their distance, speed, and direction. A radar system sends out a microwave beam, and when it hits an object, it bounces back to a receiver. Microwaves are perfect for radar because their short wavelengths reflect well off objects like cars, ships, and airplanes. Also, the antennas needed to create narrow, accurate beams are a convenient size. Radar is used in air traffic control, weather forecasting, and even to measure car speeds.

Exploring Space with Radio Astronomy

Some of the dish antennas of the Atacama Large Millimeter Array (ALMA) in Chile. This radio telescope receives microwaves from space.

Maps of the cosmic microwave background radiation (CMBR), showing how scientists have improved their view of the early universe.

Radio astronomy uses large radio telescopes to study microwaves coming from space. Planets, stars, galaxies, and nebulas all emit microwaves. By studying these signals, astronomers learn about the universe. Scientists have even bounced microwaves off planets in our Solar System to map their surfaces. A famous discovery in radio astronomy is the cosmic microwave background radiation (CMBR). This faint microwave glow fills the entire universe. It's like an echo from the Big Bang, giving us clues about how the universe began.

Heating and Power Applications

A small microwave oven on a kitchen counter, a common household appliance.

The most well-known use of microwaves for heating is in the microwave oven. These ovens use microwaves at about 2.45 GHz to heat food. The microwaves make water molecules in the food vibrate, which creates heat and cooks the food quickly. Microwave heating is also used in factories to dry and harden products. Scientists use microwaves in experimental fusion reactors to heat gases to extremely high temperatures. This helps create a plasma, which is needed for fusion energy research.

Microwaves are used for heating in many industrial processes. This is a microwave tunnel oven used to soften plastic rods.

There has also been research into using microwaves to transmit power over long distances. For example, some ideas involved using solar power satellites to beam energy from space to Earth using microwaves.

Spectroscopy

Microwaves are used in a scientific technique called electron paramagnetic resonance (EPR or ESR) spectroscopy. This method helps scientists study tiny particles called unpaired electrons in chemical systems. It gives them information about the structure and behavior of different materials.

Health and Safety with Microwaves

Microwaves are a type of non-ionizing radiation. This means that microwave energy is not strong enough to damage molecules or break chemical bonds, unlike X-rays or ultraviolet light. The word "radiation" here simply means energy moving away from a source, not radioactivity. The main effect of microwaves on materials, including our bodies, is to heat them up. The electromagnetic fields cause water molecules to vibrate, creating warmth. Most studies suggest that low levels of microwave exposure are not harmful. However, high levels of exposure can cause heat damage. The lens and cornea of the eye are especially sensitive because they don't have blood vessels to carry heat away. Strong microwave exposure can cause cataracts, which make the eye's lens cloudy. Very high doses of microwave radiation, like from a damaged microwave oven, can cause serious burns to deeper tissues in the body. It's important to always use microwave ovens safely and as intended.

The History of Microwaves

The idea of using microwave frequencies grew because lower frequency radio bands were getting crowded. Also, shorter wavelengths allowed for smaller antennas.

Early Discoveries: Hertzian Optics

Microwaves were first created in the 1890s by physicists. They thought of them as a form of "invisible light." James Clerk Maxwell had predicted in 1873 that electric fields and magnetic fields could travel together as electromagnetic waves. He suggested that light was made of these waves. In 1888, Heinrich Hertz proved Maxwell's theory. He built a simple spark gap transmitter to create radio waves. Hertz experimented with short radio waves, including those in the microwave range (50, 100, and 430 MHz).

Heinrich Hertz's spark transmitter from 1888. It used a dipole antenna and a parabolic reflector.
Jagadish Chandra Bose in 1894. He was the first to create millimeter waves.
The 3 mm spark ball oscillator Bose used to make 60 GHz waves.
$Refraction of Hertzian waves by paraffin prism$

An experiment by John Ambrose Fleming in 1897. It showed how microwaves bend when passing through a paraffin prism.
Augusto Righi's 12 GHz spark oscillator and receiver from 1895.
Oliver Lodge's 5-inch oscillator ball, used to generate 1.2 GHz microwaves in 1894.
Guglielmo Marconi's 1.2 GHz microwave spark transmitter and receiver from 1895.

Hertz showed that radio waves behaved like light. They could be refracted, diffracted, and polarized. This proved that both radio waves and light were forms of Maxwell's electromagnetic waves. Later, in 1894, Jagadish Chandra Bose conducted the first experiments with microwaves. He was the first to create millimeter waves, reaching frequencies up to 60 GHz. He also invented waveguides and horn antennas. However, early microwaves could only travel short distances in a straight line. So, scientists focused on lower frequencies for long-distance radio communication.

First Microwave Communication Experiments

Microwaves weren't widely used until the 1940s and 1950s. This was because it was hard to create high-frequency signals with the technology available then.

Antennas for an experimental 1.7 GHz microwave link across the English Channel in 1931.
An experimental 3.3 GHz transmitter from 1933, using a split-anode magnetron.
George Southworth showing a waveguide at a meeting in 1938. Microwaves passed through the flexible metal hose.
Wilmer L. Barrow with the first modern horn antenna in 1938.

In 1931, a team led by Andre C. Clavier showed the first experimental microwave relay link. It crossed the English Channel, sending phone calls and other data over 40 miles (64 km) using 1.7 GHz microwaves. This was one of the first times the term "micro-wave" was used.

Radar Development During World War II

The development of radar during World War II truly advanced microwave technology. Shorter microwave wavelengths were perfect for radar. They allowed small antennas to create narrow beams, which could accurately find enemy aircraft.

The first cavity magnetron tube, created in 1940 at the University of Birmingham.
The first commercial klystron tube, made by General Electric in 1940.
The British Mk. VIII, the first microwave air intercept radar, in the nose of a fighter plane.
A mobile US Army microwave relay station from 1945. It could transmit up to 8 phone calls.

Scientists also invented waveguides in 1936 to carry microwaves with less power loss. The horn antenna was invented in 1938 to send and receive microwaves efficiently. Key inventions during this time were the klystron tube (1937) and the cavity magnetron tube (1940). The magnetron, in particular, made powerful microwave radar possible. This technology was crucial for the Allies during the war.

Microwaves After World War II

After the war, microwaves quickly found commercial uses. Because they could carry so much information, they were perfect for long-distance communication.

In the 1950s and 60s, large microwave relay networks were built across countries to carry phone calls and TV programs. The first communications satellites, launched in the 1960s, also used microwaves to connect distant parts of the Earth. In 1964, Arno Penzias and Robert Woodrow Wilson discovered the cosmic microwave background radiation while working with a satellite antenna. This discovery was a major breakthrough in understanding the universe. Radar and satellite communication also led to the development of modern microwave antennas, like the parabolic antenna (dish antenna).

Solid-State Microwave Devices

The invention of semiconductor electronics in the 1950s brought new ways to create microwaves.

Devices like the tunnel diode (1957), IMPATT diode (1956), and Gunn diode (1962) were invented. These small, solid-state devices became widely used for generating microwaves. The maser, invented in 1953 by Charles H. Townes and his team, could amplify microwaves. Masers led to the development of atomic clocks, which keep incredibly precise time using microwave frequencies from atoms.

Microwave Integrated Circuits

Before the 1980s, microwave devices were large and expensive. But then, tiny and affordable solid-state microwave components were developed. These could be placed on circuit boards.

A microstrip circuit used in a satellite television dish. These tiny circuits process microwave signals.

This led to the creation of microstrips, which are special transmission lines on printed circuit boards. They allowed for compact microwave circuits. New transistors that could work at microwave frequencies were also developed. Materials like gallium arsenide (GaAs) were used to make faster transistors. By 1976, the first integrated circuits (ICs) that worked at microwave frequencies were created. These were called monolithic microwave integrated circuits (MMICs). Today, MMICs are found in almost all modern wireless devices, including smartphones, Wi-Fi routers, and Bluetooth devices.

Images for kids

An absorption wavemeter used for measuring microwave frequencies.