Electromagnetic radiation

In physics, electromagnetic radiation, also called electromagnetic waves, are waves that carry energy and move through space. They include many types, like radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All of these waves travel at the same speed—the speed of light—and can act like both waves and tiny particles called photons.

Electromagnetic radiation comes from moving charged particles, such as those in the Sun or machines we create. It has many important uses. For example, radio waves help us with broadcasting and wireless communication, infrared is used in thermal imaging, and visible light lets us see. Higher-energy radiation, like X-rays and gamma rays, is used in doctors’ offices to look inside our bodies and to treat sickness.

In the study of quantum mechanics, electromagnetic radiation is thought of as being made of photons. These tiny particles carry the energy of electromagnetic waves and are responsible for how light and other radiation interact with atoms and other tiny parts of matter.

Physics

Electromagnetic radiation is produced by moving charged particles and can come naturally from the Sun or be created artificially for many uses. The energy in these waves is called radiant energy. These waves do not need a material to travel through; they move through empty space at the speed of light.

Electromagnetic waves have both electric and magnetic parts that add together in specific ways. They can bend when they pass from one material to another, and they can show patterns of bright and dark areas called interference. These waves can act like both waves and tiny particles called photons, depending on how we observe them.

In empty space, electromagnetic radiation travels at the speed of light, about 186,000 miles per second. When it moves through other materials, it slows down a little. The speed at which it travels depends on properties of the material it moves through.

History of discovery

People have discovered many types of invisible light over time. In 1800, an astronomer named William Herschel found that there are warm rays just beyond red light. These are called infrared rays.

James Clerk Maxwell (1831–1879)

Soon after, in 1801, a scientist named Johann Wilhelm Ritter found even more invisible rays just beyond violet light. These are called ultraviolet rays. They can cause chemical changes.

Later, scientists learned that all these types of light are part of something called electromagnetic radiation. In the late 1800s, Heinrich Hertz made and studied radio waves. And in 1895, Wilhelm Röntgen discovered X-rays.

Finally, scientists found that some materials give off very powerful rays called gamma rays. These were identified in the early 1900s.

Electromagnetic spectrum

Main article: Electromagnetic spectrum

Electromagnetic radiation, or EMR, is energy that travels through space as waves. It includes many types, like radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All these types travel at the same speed, the speed of light.

EMR is grouped by how long its waves are. Radio waves have the longest waves, and gamma rays have the shortest. Shorter waves have more energy than longer waves. This energy can affect materials in different ways, depending on the type of EMR. For example, visible light helps us see, and X-rays can look inside the body.

Atmosphere and magnetosphere

Main articles: ozone layer, shortwave radio, skywave, ionosphere, atmospheric window, and optical window

The Earth's air helps protect us by blocking some of the Sun's strong rays. Most of the Sun's ultraviolet light and X-rays are stopped by gases like nitrogen and oxygen, and by the ozone layer high above us. Only about a third of the Sun's ultraviolet light reaches the ground, and most of it passes through safely.

We can see clearly because visible light passes through the air easily. This is called an atmospheric window. The air also lets most microwave and radio waves pass through, but certain layers high up, called the ionosphere, can bounce some radio waves, allowing us to pick up signals from far away.

Thermal and electromagnetic radiation as a form of heat

Main articles: Thermal radiation and Planck's law

When energy from electromagnetic waves hits matter, it makes the tiny parts inside the matter move faster. This energy can warm up the material or make it glow. For example, infrared waves, like those from heat lamps, are often called heat because they raise the temperature of things they touch.

All kinds of electromagnetic waves can warm materials when they are absorbed. This is why microwave ovens cook food and why intense radio waves can burn living tissue. Even visible light and ultraviolet light from very strong lasers can start fires. So, whether it’s radio waves, infrared, visible light, or ultraviolet, these waves can all add heat to materials they come into contact with.

Biological effects

Bioelectromagnetics studies how electromagnetic radiation affects living things. The impact of this radiation on living cells, including humans, depends on its strength and frequency. For lower-frequency radiation, like radio waves up to near ultraviolet, the main effect is heating when the radiation is absorbed. The frequency matters because it changes how deeply the radiation penetrates into the body—for example, microwaves go deeper than infrared light.

Ultraviolet (UV) radiation from the sun is known to cause skin cancer, the most common type of cancer in people with fair skin. UV rays damage DNA and cells and can block the body’s ability to repair this damage. While visible and infrared light might make skin look older, regular sunscreens don’t protect against these effects.

The World Health Organization has listed radio frequency electromagnetic radiation as Group 2B—meaning it might possibly cause cancer. This group includes things like lead, coffee, and car exhaust.

Use as a weapon

Some types of electromagnetic radiation can be used to create uncomfortable heat on the skin. The US military created a device called the Active Denial System to make it hard for people to enter an area by using microwave frequencies. There have also been ideas for “death rays” that use very strong electromagnetic energy to hurt people, though these are still theoretical. One inventor, Harry Grindell Matthews, said he injured his eye while working on such a device in the 1920s using a microwave magnetron—similar to what is found in a normal microwave oven.

Derivation from electromagnetic theory

Main article: Electromagnetic wave equation

Electromagnetic waves come from basic rules of electricity and magnetism called Maxwell's equations. These equations show that changing electric and magnetic fields can create waves that move through space.

Starting with Maxwell's ideas in open space, we find that these waves travel at the speed of light. This speed depends on two special numbers: one for how easily space can let electric fields pass through it, and another for how easily space can let magnetic fields pass through it.

The waves have electric and magnetic parts that move together, always at right angles to each other and to the direction the wave is moving. This is why we see light and other kinds of radiation behaving the way they do.

∇ ⋅ E = 0 {\displaystyle \nabla \cdot \mathbf {E} =0}

∇ × E = − ∂ B ∂ t {\displaystyle \nabla \times \mathbf {E} =-{\frac {\partial \mathbf {B} }{\partial t}}}

∇ ⋅ B = 0 {\displaystyle \nabla \cdot \mathbf {B} =0}

∇ × B = μ 0 ε 0 ∂ E ∂ t {\displaystyle \nabla \times \mathbf {B} =\mu _{0}\varepsilon _{0}{\frac {\partial \mathbf {E} }{\partial t}}}

∇ × ( ∇ × E ) = ∇ × ( − ∂ B ∂ t ) {\displaystyle \nabla \times \left(\nabla \times \mathbf {E} \right)=\nabla \times \left(-{\frac {\partial \mathbf {B} }{\partial t}}\right)}