Radiation is the emission or transmission of energy through space or some material in the form of a wave. This radiation spreads out from the source in all directions and "radiates" out. Radiation can also refer to the emitted energy itself. There are many different types of radiation that can include electromagnetic, thermal, acoustic, particle radiation (such as alpha or beta radiation from a radioactive source), and ionizing radiation.
Electromagnetic radiation is a type of radiation of energy that moves as a series of alternating waves and is produced by accelerating charges. This radiation is also known as an electromagnetic wave as it is composed of alternating electric and magnetic fields. This type of radiation comes in discrete packets known as photons.
There are several different types of electromagnetic radiation, and their properties depend on their energy and wavelength. Some of the different types include radio waves, infrared radiation (felt as heat), microwaves, visible light, ultraviolet radiation, x-rays, gamma rays, and cosmic rays.
Thermal radiation is one type of electromagnetic radiation that is usually talked about in great detail. In terms of heat transfer, radiation is the emission of thermal energy in the form of infrared waves. Genearlly, thermal radiation and infrared waves are referred to simply as "heat". Since heat is carried by electromagnetic waves, it does not need a physical medium to transfer energy. Instead it radiates through space - this is how the Earth is heated by the Sun despite space being a vacuum.
All objects at normal temperature emit thermal radiation; however it is not visible to the eye. Infrared cameras are able to pick up this invisible radiation and digitally convert it into a visible image (such as the one shown in Figure 1). Sometimes this radiant heat is visible. For example, a candle radiates heat. It also gives out visible light, which corresponds with the temperature of the flame. The flame burns hottest at the wick, and gives off blue or white light as it is on the higher end of the visible light spectrum. Surrounding the wick, the flame is yellow and then red in appearance, which is at the lower end of the visible light spectrum. The region surrounding the flame gives off no light as it emits infrared waves, but feels warm to the touch.
Sunlight, also called solar radiation, is a form of radiation that originated from the Sun. The radiation is part of the electromagnetic spectrum, including infrared, visible, and ultraviolet light. Sunlight that is incident upon the Earth's surface has been filtered through the atmosphere, with some of the ultraviolet radiation being absorbed. In addition to providing light for Earth, sunlight also acts as a source of radiant heat, warming the Earth.
Ionizing radiation is a specific type of radiation that has enough energy to eject an electron from some atom. This radiation includes ionizing particles from alpha or beta decay, as well as electromagnetic waves in the form of gamma radiation. Generally speaking, the energies of alpha and beta decay particles and gamma ray photons is higher than the ionization energies of atoms and molecules. These particles ionize the matter and break molecular bonds, which can cause significant biological damage.