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The word light usually makes one think of the colors of the rainbow or light from the Sun or a lamp. Operated for NASA by SAOThis site was developed with funding from NASA under Contract NAS8-03060. Visible light is a complex phenomenon that is classically explained with a simple model based on propagating rays and wavefronts, a concept first proposed in the late 1600s by Dutch physicist Christiaan Huygens.
The term electromagnetic radiation, coined by Sir James Clerk Maxwell, is derived from the characteristic electric and magnetic properties common to all forms of this wave-like energy, as manifested by the generation of both electrical and magnetic oscillating fields as the waves propagate through space.
The link between light, electricity, and magnetism was not immediately obvious to early scientists who were experimenting with the fundamental properties of light and matter. Ultraviolet radiation, at the other end of the visible spectrum, was discovered by Wilhelm Ritter, who was one of the first scientists to investigate the energy associated with visible light. Explore wave propagation through space with a sinusoidal representation of electromagnetic radiation. Electricity and magnetism were first associated in 1820 when Danish physicist Hans Christian Oersted discovered that electrical current flowing through a wire could produce deflections of a compass needle.
Finally, in 1865, Scottish scientist James Clerk Maxwell expanded his kinetic theory of gases to mathematically explain the links between electricity and magnetism.
An electromagnetic wave travels or propagates in a direction that is oriented at right angles to the vibrations of both the electric (E) and magnetic (B) oscillating field vectors, transporting energy from the radiation source to an undetermined final destination. Whether taking the form of a signal transmitted to a radio from the broadcast station, heat radiating from a fireplace, the dentist's X-rays producing images of teeth, or the visible and ultraviolet light emanating from the sun, the various categories of electromagnetic radiation all share identical and fundamental wave-like properties. The classical schematic diagram of an electromagnetic wave presented in Figure 2 illustrates the sinusoidal nature of oscillating electric and magnetic component vectors as they propagate through space. A standard measure of all electromagnetic radiation is the magnitude of the wavelength (in a vacuum), which is usually stated in units of nanometers (one-thousandth of a micrometer) for the visible light portion of the spectrum. The hertz was designated as a standard unit of electromagnetic radiation frequency in recognition of the work of German physicist Heinrich Hertz, who succeeded in generating and performing experiments with electromagnetic waves in 1887, eight years after the death of Maxwell.
Examine how the frequency, energy, and wavelength of an electromagnetic wave are related, and how changing one automatically produces a corresponding change in the other two (and the color of visible light). The different wavelengths and frequencies comprising the various forms of electromagnetic radiation are fundamentally similar in that they all travel at the same speed—about 186,000 miles per second (or approximately 300,000 kilometers per second), a velocity commonly known as the speed of light (and designated by the symbol c).
Under normal circumstances, when traveling through a uniform medium, such as air or a vacuum, light propagates in straight lines until interaction with another medium or material induces a path change, through refraction (bending) or reflection. Visible light displays classical wave-like properties, but it also exhibits properties reminiscent of particles, which are manifested through entities that possess energy and momentum (but no mass), and are referred to as photons.


Eventually, the "excited" electron loses the extra energy by emitting electromagnetic radiation of lower energy and, in doing so, falls back into its original and stable energy level.
Electromagnetic radiation energy levels can vary to a significant degree depending upon the energy of source electrons or nuclei.
Even though electromagnetic radiation is customarily described by the wavelength and frequency of the waveforms, other characteristic properties are important when considering how waves propagate through space. When light is non-polarized (Figure 4), the electric field vectors vibrate in all planes lying perpendicular to the direction of propagation.
Most artificial and natural light sources emit non-coherent light that displays a variety of phase relationships among the wavelengths present in the spectrum (Figure 4). Light waves that have coaxial, relatively non-diverging paths as they travel through space are termed collimated. Gamma rays - High-energy radiation that possesses the highest frequency (and shortest wavelengths), gamma rays are emitted as the result of transitions within the atomic nucleus, including nuclei of certain radioactive materials (natural and artificial). Exposure to gamma rays can induce mutations, chromosome aberrations, and even cell death, as is often observed in some forms of radiation poisoning. X-rays - Electromagnetic radiation having a frequency just above the ultraviolet (but below the gamma) range is classified as X-rays, and is energetic enough to pass easily through many materials, including the soft tissues of animals. Hot gases in outer space emit a powerful spectrum of X-rays, which are utilized by astronomers to gain information about the origin and characteristics of interstellar regions of the universe. Ultraviolet Light - Often abbreviated (uv), ultraviolet radiation propagates at frequencies just above those of violet in the visible light spectrum.
Ultraviolet light is utilized extensively in scientific instruments to probe the properties of various chemical and biological systems, and it is also important in astronomical observations of the solar system, galaxy, and other parts of the universe. Some insects (notably honeybees) and birds have sufficient visual sensitivity in the ultraviolet region to respond to longer wavelengths, and may rely upon this capability in navigation. The low-energy, long-wavelength red colors (622-780 nanometers) are followed in sequence by orange (597-622 nanometers), yellow (577-597 nanometers), green (492-577 nanometers), blue (455-492 nanometers), and finally, relatively high-energy, short-wavelength violet (455 nanometers and below). Division of the visible light spectrum into color regions based on physical properties is straightforward, but the manner in which color is sensed is not as obvious.
Visible light is the basis for all life on Earth, and is captured by primary producers or autotrophs, such as green plants. In 1672, Sir Isaac Newton studied the interaction of visible light with a glass prism and first recognized that white light is actually a mixture of different colors representing the entire visible light spectrum. Infrared Radiation - Often abbreviated IR, the large band of infrared wavelengths extends from the far-red portion of the visible light spectrum (around 700-780 nanometers) to about one millimeter in wavelength.


Molecules of all objects that exist above the temperature of absolute zero (-273 degrees Celsius) emit infrared rays, and the amount of emission generally increases with temperature. Common tools that rely on detection of infrared radiation are night vision scopes, electronic detectors, sensors in satellites and airplanes, and astronomical instrumentation.
Infrared photography takes advantage of the near-infrared spectrum to record images on specialized film useful in forensics, remote sensing (such as aerial crop and forest surveys), painting restorations, satellite imaging, and military surveillance applications.
Microwaves - Currently the basis of a widespread technology utilized in millions of households for heating food, microwave spectral wavelengths range from approximately one millimeter to thirty centimeters (or about one foot). Discover how a variable capacitor is coupled to a simple antenna transformer circuit to tune specific wavelengths in a broadband radiofrequency spectrum. Microwaves constitute the highest frequency radio waves, and are emitted by the Earth, buildings, cars, airplanes, and other large objects. Microwaves are also employed for transporting information from Earth to orbiting satellites in vast communications networks, for relaying information from ground-based stations over long distances, and in terrain mapping. Radio Waves - The expansive radiofrequency portion of the electromagnetic spectrum includes wavelengths from about 30 centimeters to thousands of kilometers.
The sound and video portion of television is transported through the atmosphere by shorter radio waves having wavelengths less than a meter, which are modulated for broadcast much like FM radio. The nature of the relationship between the frequency (the number of oscillations per unit time) and wavelength (the length of each oscillation) of light becomes apparent when studying the broad range of the electromagnetic radiation spectrum. The spectrum consists of radiation such as gamma rays, x-rays, ultraviolet, visible, infrared and radio. The energy of the radiation depends on the distance between the crests (the highest points) of the waves, or the wavelength.
Gamma rays have wavelengths less than ten trillionths of a meter which is about the size of the nucleus of an atom. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.




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