Radiation is, in fact, energy that travels in space. More specifically, electromagnetic radiation can be defined as being composed of particles without mass (also called photons) which travel at the speed of light in a wave-like pattern. It is the energy of those photons that is used as criteria to classify various types of electromagnetic radiation. Thus, there are various types of radiations, all being electromagnetic radiation. The following is the increasing order of wavelength and decreasing order of energy and also of frequency between various types of electromagnetic radiation:
Gamma-rays have the highest energy and frequency and the lowest wavelength. They are emitted by radioactive materials when they undergo nuclear fission such as through the detonation of nuclear weapons and by the generation of power in nuclear power plants.
X-Rays are used in medicine and may lead to workplace exposure. In the universe, hot gases may emit X-Rays.
Ultraviolet light is emitted by the sun and other hot objects in the space;
Visible light – that is the only radiation type that we can perceive. It is emitted by everything: objects and living beings, plants, and even by stars! This is why we see what we see.
Infrared light is emitted by the human body (specifically by skin), which is why we can be seen in the night by using night-vision devices (goggles). Infrared light is also used in space research to map the dust between stars.
Microwaves are emitted in the microwave ovens and by radars. They are used in space research to study the structure of galaxies.
Radio waves have the lowest energy and frequency but the highest wavelength. They are emitted by radio transmitters, TVs, etc. The radio waves are also emitted by stars and gases in the Universe and may be used in astronomy research.