Electromagnetic waves are a form of energy that manifest themselves in a variety of forms, including radio waves, light waves, microwaves, infrared radiation, ultraviolet radiation, X rays, and gamma rays. This range of electromagnetic phenomena is generally known as the electromagnetic spectrum.
Electromagnetic waves are producing by vibrating electric charges. The number of vibrations per second is called the frequency of the wave and is measured in hertz (Hz; cycles per second). The lowest frequencies observed in electromagnetic waves are about 30 hertz. The highest frequency waves are those of gamma rays, with a frequency of about 1019 (10,000,000,000,000,000,000) hertz. Light waves have a frequency of about 1015 hertz.
Wavelength is the distance between successive crests or troughs of a wave. Wavelengths are measured in nanometers, millimeters, centimeters, meters, or kilometers, depending on the type of wave. Radio waves have the longest wavelengths, ranging from about 10,000 kilometers (6,000 miles) to about one millimeter (0.04 inch). By contrast, the wavelength of gamma rays is about 10-12 meters, smaller than the diameter of an atom.
The velocity (speed) of all electromagnetic waves is equal to the frequency times the wavelength. That product is a constant, equal to about 300,000,000 meters per second (186,000 miles per second).
Radio waves are defined as electromagnetic waves with wavelengths greater than one millimeter and are subdivided into a number of subcategories, depending on their wavelengths. These categories include ultra high frequency (UHF), very high frequency (VHF), high frequency (HF), medium frequency (MF), low frequency (LF), very low frequency (VLF), and extremely low frequency (ELF) waves.
Infrared waves are located between radio waves and visible light. Radio waves that are shorter than one meter are called microwaves, but they share some properties with infrared wavelengths. Infrared radiation is observed as heat.
Visible light has wavelengths ranging from about 390 nanometers (nm) to 750 nm. Light waves of different wavelengths are perceived as having different colors by the eye. For example, red light has wavelengths of about 700 nm; green light, about 550 nm; and violet light, about 400 nm. Infrared radiation consists of waves with wavelengths greater than 750 nm, while ultraviolet radiation consists of waves with wavelengths from about 1 to about 400 nm.