The electromagnetic spectrum consists of the totality of all electromagnetic radiation. Made up of photons, everything in the electromagnetic spectrum is sometimes referred to as light, although the word sometimes refers to only the human-visible portion of the electromagnetic spectrum.
Photons have some characteristics of a particle and some of a wave. For example, they have a wavelength. The wavelength of a photon ranges from many times smaller than an atomic diameter to the width of the earth. All radiation that is part of the electromagnetic spectrum has three fundamental properties – frequency or wavelength, intensity, and polarization. The last property, which refers to the angle of the electromagnetic wave, cannot be detected by the human eye, although bees can perceive it.
All electromagnetic radiation travels at the speed of light, although an intervening material of the right makeup can reflect it, slow it, or absorb it. The primary source of electromagnetic spectrum radiation on earth is from the sun, though many processes can create it, including various phosphorescent life forms.
Electromagnetism may be thought of as an oscillating perturbation in the electromagnetic field. The electromagnetic spectrum consists of all possible perturbations. We can only see a small portion of them – light with a wavelength between about 400 nanometers and about 750 nanometers. Perhaps not surprisingly, this is the wavelength of light that pours in large quantities from the sun, and our visual systems have evolved to perceive it. The acronym ROYGBIV is sometimes used to describe the human-visible colors of the electromagnetic spectrum, in order of lowest frequency to highest frequency: red, orange, yellow, green, blue, indigo, and violet.
The type of electromagnetic radiation with a wavelength longer than that of the color red is called infrared, and is given off by all objects that release heat. Infrared ranges in wavelength from about 750 nanometers to as long as a millimeter. Following infrared are microwaves, with a wavelength between about a millimeter to around 30 centimeters (12 inches). These are used in a microwave oven. Radio waves include anything with a longer wavelength than microwaves. These have the greatest ability to penetrate the earth’s atmosphere and therefore are extremely important to communications technology.
Electromagnetic radiation with smaller wavelengths than visible light include the ultraviolet, followed by x-rays, then gamma rays. Gamma rays are a type of cosmic ray and can have extremely high energies. Gamma rays can have energies much higher than anything yet produced in our particle accelerators, and their wavelengths can be as small as a single subatomic particle.