An emitter is a device that emits radiation. The term can refer to either natural or artificial sources of radiation. Natural emitters include the Sun, stars, and radioactive materials found in the Earth’s crust. Artificial emitters include X-ray machines, lasers, and light-emitting diodes (LEDs).
Radiation is a type of energy that travels through space in the form of waves or particles. Waves are classified according to their wavelength, which is the distance between two peaks of the wave. The shorter the wavelength, the more energetic the radiation. Particles are classified according to their mass and velocity. The more massive and fast-moving a particle is, the more energetic it is.
The amount of energy emitted by an object depends on its temperature. Hotter objects emit more energetic radiation than cooler objects. This relationship is known as Wien’s law. The hottest known object in the Universe is the cosmic microwave background radiation, which has a temperature of 2.7 Kelvin (-270°C).
Radiation can be harmful to living things. Ionizing radiation—that is, radiation with enough energy to strip electrons from atoms—can damage DNA and cause cancer. Non-ionizing radiation—such as visible light and radio waves—is less harmful but can still be harmful if exposure is high enough (for example, if you stare at the Sun without wearing proper eye protection).