In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.

This includes:
 electromagnetic radiation such as radio waves, visible light, and x-rays
 particle radiation such as α, β, and neutron radiation
 acoustic radiation such as ultrasound, sound
 seismic waves.

Radiation may also refer to the energy, waves, or particles being radiated. Originally, radiation waves do not contain particles as they are transferred to Earth by the Sun for example.

Electromagnetic radiation 
Many people are already familiar with electromagnetic radiation (EMR), including light.  The electromagnetic spectrum shows the types of radiation according to their wavelength and frequency. Some kinds are:
 Ionizing radiation comes from radioactive materials and X-ray machines and non-ionizing radiation comes from other sources. Ionizing radiation carries more than 10 eV (electronvolts), which is enough to ionize atoms and molecules, and break chemical bonds. This is important for its harmfulness to living organisms. Non-ionizing radiation does not cause microscopic damage, but it can make things hotter and some types can cause chemical changes.
 X-rays and gamma rays: These very strong rays are commonly used in medicine to photograph the interior of the body and treat cancer. However, in large amounts, they are dangerous to life.
 Ultraviolet light: This is a type of radiation with more energy than visible light. It gives people a sunburn. Ultraviolet light is used to kill bacteria.
 Visible light: This is the radiation that we see all around us as what most people call ‘light’. It can make chemical changes.
 Infrared waves: Objects at room temperature emit infrared radiation. Although humans cannot see it, special cameras can pick up this kind of radiation.
 Radio waves: This is the kind of electromagnetic radiation with the longest waves.  Radio waves are used to send and receive communications.
 Micro-waves: This kind of radio wave is used by a microwave oven to warm up food. Microwaves are also used for communications, as weapons, and to move electrical power from one place to another.
 Radar waves: This kind of radio wave is used to detect air planes in the sky and ships in the ocean. Radar is also used to see changes in weather.

Danger from radiation 
Ionizing radiation is radiation that carries enough energy to free electrons from atoms or molecules.

Only certain types of radiation are harmful to humans. For example, ultraviolet radiation can give people sunburns. X-rays and gamma rays can make a person sick, or even die, depending on the dose they get. Some types of particle radiation can also make people sick and lead to burns. If radiation does not carry high enough levels of energy, though, then these changes will not happen when something is hit by the radiation. This is referred to as non-ionizing radiation, which is not as dangerous.

One can distinguish between various types of radiation by looking at the source of the radiation, its wavelength (if the radiation is electromagnetic), the amount of energy being carried, any particles involved, etc. Radioactive material is a material which emits radiation. Uranium and plutonium are examples of radioactive materials. The atoms they are made of tend to fall apart and give off different kinds of radiation, like gamma rays and lots of particle radiation.

Ionizing radiation by type 
Ionizing radiation can kill living things. It can cause genetic mutations, as shown by H.J. Muller. It can destroy cells in the body which divide, and thus indirectly kill a person.
 Alpha radiation, a type of particle radiation made up of the nuclei of helium atoms.
 Beta radiation, another type of particle radiation made up of high energy electrons or positrons.
 Neutron radiation, yet another type of particle radiation made up of high energy neutrons.
 Gamma radiation (Gamma rays), a type of radiation made up of high energy photons.
 X-ray radiation (X-rays), a type of radiation also made up of photons but which typically contain less energy than gamma rays.

Non-ionizing radiation by type 
 Ultraviolet radiation, also known as UV.
 Visible light
 Infrared radiation
 Radio waves including
 Microwave radiation
 Gravitational radiation, a predicted consequence of general relativity.
 Sound waves

Related pages
 Background radiation
 Cosmic microwave background radiation, 3K blackbody radiation that fills the Universe
 Laser
 Maser
 Radiation damage – destructive effects on materials and devices
 Radiation poisoning – destructive effects on life forms
 Radiation hardening – making devices resistant to failure in high radiation environments
 Radioactive contamination
 Radioactive decay
 Radiation accidents

References

Basic physics ideas
Wave physics
Nuclear physics