The electromagnetic spectrum is the name we use when we talk about different types of radiation as a group. The parts of the electromagnetic spectrum, arranged from highest energy to lowest, are gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves.
All the parts of the electromagnetic spectrum are the same thing — radiation. Radiation is made up of a stream of photons — particles without mass that move in a wave pattern, all at the same speed — the speed of light. Each photon contains a certain amount of energy.
The only difference between the parts of the electromagnetic spectrum is the amount of energy the photons contain. Radio waves have the least energy, and gamma rays have the most. Low-energy radiation has long waves, and high-energy radiation has short waves. Infrared radiation is a form of radiation with less energy than visible light, but more than microwaves. When an object is not hot enough to emit visible light, it will emit most of its energy in infrared.
Infrared also covers a range of wavelengths, so scientists discuss such categories as “near infrared,” the range closest to visible light, and “far infrared,” the range closest to microwave radiation. When human beings experience the sensation of heat from an object emitting infrared, we are detecting near or mid infrared radiation. Near and mid infrared can penetrate plastics and other materials, and comes in handy for things like controlling the television with a remote control.
Notice, in the electromagnetic spectrum, that visible light is positioned right next to infrared light. When the expansion of the universe stretches visible light, it turns that short-wave, higher-energy light into longer-wave, lower-energy infrared light. This is known as cosmological redshift.