GammaRays

X-Rays

Ultraviolet

Visible

Infrared

Microwaves

Radio Waves

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The Electromagnetic Spectrum Light is just one portion of the various electromagnetic waves flying through space.  The electromagnetic spectrum covers an extremely broad range, from radio waves with wavelengths of a meter or more, down to x-rays with wavelengths of less than a billionth of a meter.  Optical radiation lies between radio waves and x-rays on the spectrum, exhibiting a unique mix of ray, wave, and quantum properties. At x-ray and shorter wavelengths, electromagnetic radiation tends to be quite particle like in its behavior, whereas toward the long wavelength end of the spectrum the behavior is mostly wavelike.  The visible portion occupies an intermediate position, exhibiting both wave and particle properties in varying degrees. Like all electromagnetic waves, light waves can interfere with each other, become directionally polarized, and bend slightly when passing an edge.  These properties allow light to be filtered by wavelength or amplified coherently as in a laser. In radiometry, light’s propagating wavefront is modeled as a ray traveling in a straight line.  Lenses and mirrors redirect these rays along predictable paths.  Wave effects are insignificant in an incoherent, large scale optical system because the light waves are randomly distributed and there are plenty of photons. The table below describes the different radiations of the Electromagnetic Spectrum. I must emphasise that these radiations are all the same except for the difference in wavelength. They have different names because of historical reasons and the way they are generated. The boundaries between the different radiations are all artificial. As you progress from Radio Waves through to Gamma Rays, the wavelength gets shorter (so they become more penetrating), the frequency gets higher (so the oscillation needed to produce them gets faster), and the energy gets higher (so it takes more energy to produce Gamma Rays than it does to produce Radio Waves). Name Wavelength (m) Frequency (Hz) Energy (J) Radio Waves 104 - 10-3 103 - 1010 10-30 - 10-23 Infra Red 10-3 - 10-6 1010 - 1014 10-23 - 10-19 Visible 10-6 1014 10-19 Ultra Violet 10-6 - 10-8 1014 - 1016 10-19 - 10-17 X-Rays 10-8 - 10-10 1016 - 1019 10-17 - 10-14 Gamma Rays 10-10 - 10-14 1019 - 1024 10-14 - 10-10 All matter produces radiation. Radio Waves are produced when free electrons are forced to move in a magnetic field, or when electrons change their spin in a molecule. They are used for communication and to study low energy motions in atoms. All electrical goods generate Radio Waves. Radio Waves from space can be used to study cool interstellar gases. Radio Waves cannot be detected by humans. Infra Red radiation is produced by the vibrations of molecules. Human skin feels this radiation as heat. Microwave ovens work by using Infra Red radiation of the correct frequency to make the water molecule vibrate faster. A faster vibrating molecule is a hotter molecule. Only the food which contains water is affected. The plate which is a dry mineral is unaffected. Infra Red is used as an analytical tool for molecules in Chemistry. Cool, proto-stars are studied with Infra Red detectors. Visible and Ultra Violet Light is produced by chemical reactions and ionisations of outer electrons in atoms and molecules. There are many chemical reactions that are instigated by this radiation: the chemical retinal in animal eyes, chlorophyll in plants, silver chloride in photography, the chemical melanin in human skin, silicon converts light to electricity. Light is the most familiar electromagnetic radiation because the Earth's atmosphere is transparent to it. Light (and a little of the Infra Red and Ultra Violet on either side of it) can pass through the atmosphere. Living organisms have evolved to use these waves. Visible Light is simply the part of the electromagnetic spectrum that reacts with the chemicals in our eyes. Bees can see more Ultra Violet than we can. Snakes can detect Infra Red. X-Rays are produced by fast electrons stopping suddenly, or by ionization of the inner electrons of an atom. They are produced by high energy processes in space: gases being sucked in to a black hole and becoming compressed; exploding stars. They are used in medicine to look through flesh. In Physics the waves are small enough to pass between atoms and molecules so they can be used to determine molecular structures. Gamma Rays are produced by very high energy processes, usually involved with the nucleus of atoms. Radioactivity and exploding stars produce Gamma Rays. They are very dangerous because if they strike atoms and molecules they will do lots of damage. If the molecules are the long and complex molecules of life, death and mutation could occur. Information provided by:http://www.krysstal.com Ultraviolet Light Short wavelength UV light exhibits more quantum properties than its visible and infrared counterparts.  Ultraviolet light is arbitrarily broken down into three bands, according to its anecdotal effects. UV-A is the least harmful and most commonly found type of UV light, because it has the least energy.  UV-A light is often called black light, and is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light - thus appearing to glow in the dark.  Most phototherapy and tanning booths use UV-A lamps. UV-B is typically the most destructive form of UV light, because it has enough energy to damage biological tissues, yet not quite enough to be completely absorbed by the atmosphere.  UV-B is known to cause skin cancer.  Since most of the extraterrestrial UV-B light is blocked by the atmosphere, a small change in the ozone layer could dramatically increase the danger of skin cancer. Short wavelength UV-C is almost completely absorbed in air within a few hundred meters.  When UV-C photons collide with oxygen atoms, the energy exchange causes the formation of ozone.  UV-C is almost never observed in nature, since it is absorbed so quickly.  Germicidal UV-C lamps are often used to purify air and water, because of their ability to kill bacteria. Information provided with permission by: International Light Technologies (ILT) Visible Light Photometry is concerned with the measurement of optical radiation as it is perceived by the human eye.  The CIE 1931 Standard Observer established a standard based on the average human eye response under normal illumination with a 2° field of view.  The tristimulus values graphed below represent an attempt to describe human color recognition using three sensitivity curves.  The y(l) curve is identical to the CIE V(l) photopic vision function.  Using three tristimulus measurements, any color can be fully described. Color Models Most models of perceived color contain three components: hue, saturation, and lightness.  In the CIE L*a*b* model, color is modeled as a sphere, with lightness comprising the linear transform from white to black, and hues modeled as opposing pairs, with saturation being the distance from the lightness axis.  Information provided with permission by: International Light Technologies (ILT) Infrared Light Infrared light contains the least amount of energy per photon of any other opticalband.  Because of this, an infrared photon sometimes lacks the energy required to pass the detection threshold of a quantum detector.  Infrared is usually measured using a thermal detector such as a thermopile, which measures temperature change due to absorbed energy. While these thermal detectors have a very flat spectral responsivity, they suffer from temperature sensitivity, and usually must be artificially cooled.  Another strategy employed by thermal detectors is to modulate incident light with a chopper.  This allows the detector to measure differentially between the dark (zero) and light states. Quantum type detectors are often used in the near infrared, especially below 1100 nm.  Specialized detectors such as InGaAs offer excellent responsivity from 850 to 1700 nm.  Typical silicon photodiodes are not sensitive above 1100 nm.  These types of detectors are typically employed to measure a known artificial near-IR source without including long wavelength background ambient. Since heat is a form of infrared light, far infrared detectors are sensitive to environmental changes - such as a person moving in the field of view.  Night vision equipment takes advantage of this effect, amplifying infrared to distinguish people and machinery that are concealed in the darkness. Infrared is unique in that it exhibits primarily wave properties.  This can make it much more difficult to manipulate than ultraviolet and visible light.  Infrared is more difficult to focus with lenses, refracts less, diffracts more, and is difficult to diffuse.  Most radiometric IR measurements are made without lenses, filters, or diffusers, relying on just the bare detector to measure incident irradiance. Information provided with permission by: International Light Technologies (ILT)