Electromagnetic radiation

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.

Physics

Electromagnetic radiation is made by moving charged particles. It can come from the Sun or be made for many uses. The energy in these waves is called radiant energy. These waves can travel through empty space at the speed of light.

Electromagnetic waves have electric and magnetic parts. They can bend when they go from one material to another. They can also 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 look at them.

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

History of discovery

People have found many kinds of invisible light over time. In 1800, an astronomer named William Herschel discovered 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 more invisible rays just beyond violet light. These are called ultraviolet rays. They can cause changes in chemicals.

Later, scientists learned that all these kinds 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 moves through space as waves. It includes many types, like radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All these types move 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. 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 from the Sun's strong rays. Gases like nitrogen and oxygen, and the ozone layer, block most of the Sun's ultraviolet light and X-rays. Only a small part of the ultraviolet light reaches the ground, and most of it is safe.

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. Some radio waves can bounce off layers high up called the ionosphere, helping us receive 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 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. 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 looks at how electromagnetic radiation affects living things. How this radiation affects 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 goes into the body—for example, microwaves go deeper than infrared light.

Ultraviolet (UV) radiation from the sun can cause skin cancer. UV rays can damage DNA and cells. 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.

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. 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 rules show that changing electric and magnetic fields can make waves that travel through space.

Starting with Maxwell's ideas, we learn these waves move at the speed of light. This speed depends on two special numbers about space and how it lets electric and 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 light and other kinds of radiation act 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)}