Introduction

The term light is usually taken to refer to visible light, the familiar spectrum of red, orange, yellow, green, blue, violet. However, light actually belongs to a much broader spectrum known as the electromagnetic spectrum. It includes, in order of increasing frequency, radio waves, infrared waves, visible light, ultraviolet rays, X rays, and gamma rays.

The Electromagnetic Spectrum

The electromagnetic spectrum is the range of wavelengths and frequencies that electromagnetic radiation can assume. This is a very broad range, and these waves exhibit a variety of properties associated with wavelength and frequency.

Long radio waves have the lowest frequency and wavelength - they sometimes have frequencies less than 1 Hertz and wavelengths in excess of 1 kilometer. They are generally used for long-range radio transmissions. Short radio waves have higher frequencies and correspondingly shorter wavelength; they are used mostly in very short-range radio transmissions. AM (amplitude modulation) radio waves have frequencies between these two wave types. In varying AM waves, the strength or height (maximum displacement from equilibrium) is changed. By contrast, FM (frequency modulation) radio waves usually have higher frequencies closer to those of TV transmissions. In varying FM waves, the frequency of the wave is changed. Exposure to radio waves causes no major health problems and is not regulated.

Microwaves are higher-frequency waves lying roughly between radio and infrared waves. They have a number of common applications, the most familiar of which is the microwave or microwave oven used for cooking. In these kitchen devices, microwaves are used to excite the water molecules in food, thus generating heat. Microwaves can easily penetrate nonmetal containers but generally cannot penetrate metal. For this reason, food to be microwaved cannot be heated in metal containers. High densities of microwave radiation (such as what is found in masers, or "microwave lasers") are known to cause health problems such as burns, cataracts, nervous-system damage, and sterility. Exposure to microwave radiation is usually regulated; the U.S. government limits exposure to 10 milliwatts per square centimeter or less.

Infrared radiation is the portion of the electromagnetic spectrum just below red light in terms of frequency. Infrared radiation, along with visible light and ultraviolet rays, are produced by the transitions of outer electrons. Infrared radiation has many applications in the field of astronomy because earth's atmosphere does not scatter it as much as visible light. Thus, special filters that block all but infrared rays can be used to obtain precise astronomical images without the scattering associated with visible light. Infrared radiation can also be used in detecting the positions of objects or people in the absence of visible light. This property has been put to good use in modern military technology. A more mundane use of infrared light can be found in the admissions booths of many theme parks, where visitors' hands are stamped with special ink visible only under infrared lights to prove that admission fees have been paid. A special infrared light, often referred to as a black light, is used to detect the ink. Infrared radiation itself is also often called "black light." Few if any dangerous side effects result from low-level exposure to infrared radiation.

Visible light is what is generally referred to by the term "light." This is the only type of electromagnetic radiation detectable by human eyesight. White light can be broken up into six distinct colors, each corresponding to a separate frequency and wavelength. These colors are, in order of increasing frequency, red, orange, yellow, green, blue, violet. (Indigo is often considered the seventh color of the spectrum, but is no longer recognized as a distinct spectral color.) This spectrum can be obtained by passing white light through a prism; when it occurs naturally as a result of light reflection in water droplets, it is called a rainbow. The colors seen in everyday life are due to the disproportionate absorption of certain wavelengths by everyday objects. For example, if an object is green, it tends to absorb red, orange, yellow, blue, and violet light, but reflects green light back to the observer. If an object is a color "in between" two spectral colors - i.e., teal - then it reflects these two colors while absorbing the others. In the case of teal, red, orange, yellow, and violet light is absorbed, while green and blue light is reflected. Aside from its ordinary applications, visible light spectra are can be used to detect such things as changes in the configurations of molecules.

Ultraviolet light is just beyond violet light in terms of frequency. Its main natural source is the sun and other stars; artificially, it is produced by electric-arc lamps for scientific purposes. Ultraviolet rays are often harmful to plants and animals, including humans. Their danger is generally proportional to their wavelength. They are divided into three categories: UV-A, UV-B, and UV-C. UV-A has the longest wavelength and is least dangerous; UV-B is of intermediate wavelength and is the type of sun emission that causes sunburn and, over long periods of exposure, skin cancer; UV-C has a very short wavelength and kills bacteria and viruses so well that it is often used to sterilize surfaces. The earth's atmosphere, especially the ozone layer, provides some protection from harmful UV rays from the sun; however, the depletion of the ozone layer in recent years has led to an increase in the amount of ultraviolet radiation to which the average human is exposed. Also, ultraviolet radiation is not entirely harmful because vitamin D is produced when it hits a human's or animal's skin. Another interesting property of ultraviolet light is the fact that it causes some objects to glow, or become fluorescent, upon contact. Molecules in the object gain energy on contact with ultraviolet light, then release the energy in the form of visible light. In astronomy, satellite-based ultraviolet ray detectors provide excellent data on distant stars.

X rays, also known as Roentgen rays in honor of their discoverer, are divided into two categories: soft and hard X rays. Soft X rays have longer wavelengths and are closer to the ultraviolet band of the spectrum. Hard X rays are closer to the gamma-ray band of the spectrum and have much shorter wavelengths. X rays are produced when high-velocity electrons are hit by material objects. Each element has a certain spectrum of characteristic X rays associated with it that identify it absolutely. This is extremely useful when studying the elemental makeup of distant objects. X rays are highly penetrating of ordinary objects, and their penetration power depends on the density and atomic weight of the object. They find their best-known use in medicine, where they easily penetrate flesh and are more effectively absorbed by bone. The result is that bone appears white on a photographic plate, while soft tissues appear gray. Another related, familiar application of X rays is luggage scanning at airports and other such facilities. Again, the empty portions of luggage or light objects like clothing are easily penetrated by X rays, while other, harder objects made of metal or hard plastic absorb the radiation more effectively. X rays are also associated with ionization and research into quantum mechanics; more information on these topics is available in Theoretical Cosmology.

Gamma rays are the shortest-wavelength, highest-frequency type of electromagnetic radiation. They are essentially identical to X rays in their effect, but are produced by excited nuclei instead of inner electrons. They are the most penetrating of all electromagnetic radiation. They are often produced as a result of gamma decay of radioactive elements; this is the most dangerous and the most penetrating of all radioactive decay.

The Nature of Light

The different constituents of the electromagnetic spectrum are actually the same type of radiation with the wavelength and and frequency varied. All electromagnetic radiation is light and therefore travels at the speed of light, about 299 792 458 m/s (299 972 km/s). All electromagnetic radiation is also a manifestation of the electromagnetic force and comes in packets or quanta called photons, which are the "smallest denomination" of light. This led to the modern particle-wave picture of light, which developed from two differing ideas of light as a particle and a wave. (Note:The following discussion focuses on visible light, but applies to all forms of electromagnetic radiation.)

	The dual nature of light was determined in the now-famous double-slit experiment, in which a beam of light is shone on a panel into which two slits have been cut, exposing a photographic plate. If only one slit is open, one area on the plate is light.
	According to the particle theory, championed by Newton, two open slits should simply produce two light areas on the plate.
	According to the wave theory, supported by Christian Huygens, two open slits would produce an interference pattern. This was the actual appearance of the plate, so light was accepted to be a wave and Maxwell later wrote the equations governing the pattern.

However, Einstein's theories resurrected the particle view of light with the introduction of the idea of photons. (This is embodied by the photoelectric effect: the phenomenon in which electrons escape from a metallic surface when light is shone on it, thus implying that light is a particle.) This brought a problematic question: how can particles interfere? It was shown that even if photons were fired one by one at the panel, an interference pattern was still produced. This seemed to be an impossibility because particles, fired at the panel one by one, have nothing with which to interfere. Nevertheless, light has the properties of both a wave and a particle, leading to the quantum principle of wave-particle duality.

Later work showed that matter particles also have wave-like properties that remain unnoticed at the macroscopic level, but become very evident at the microscopic level. An experiment similar to the one described above, with the substitution of electrons for photons, was carried out and found to have the same results. This leads to the idea that electrons and other particles do not have a definite position, but instead have associated probabilities that establish the most likely places that the electron will be found. An elaboration of this idea can be found in The Uncertainty Principle.