Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye (about 400-700 nm), or perhaps 380-750 nm. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. Three primary properties of light are:

  • Intensity
  • Frequency or wavelength
  • Polarization

Light, which exists in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave-particle duality. The study of light, known as optics, is an important research area in modern physics.

Sources of light

Light SourceThere are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". Blue thermal emission is not often seen. The commonly seen blue colour in a gas flame or a welder's torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm).

Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself-so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.

Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.

Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.

Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions.

Certain other mechanisms can produce light:

  • scintillation
  • electroluminescence
  • sonoluminescence
  • triboluminescence
  • Cherenkov radiation

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:

  • Radioactive decay
  • Particle-antiparticle annihilation

Cold light sources

High intensity light is used in conjunction with endoscopes, rigid scopes, headlamps and operating microscopes etc.

Physiology: Cold light sources are normally used to illuminate a specific work area during surgery or medical examinations. It is important that the light is as near in colour to natural light so doctors are able to detect abnormal colourations of the skin and tissue. It is also important that the light does not heat the tissue.

How it works:

Medical Light SourcesThe high intensity light source consists of a power supply, cooling fan, brightness controls, lamp life indicator and a spare lamp to enable procedures to be finished if the primary lamp fails.  Filters are also employed to reduce the transmission of excessive heat to the operating site. This is why it is called 'Cold Light'.   The two main types of lamps used are xenon arc and halogen. Some xenon lamps have a useable life of only 500 hours. There is very high atmospheric pressure inside the xenon lamp, it is recommended that when replacing Xenon lamps, technicians never touch the lamp without wearing gloves and mask and also don't press the lamp (to prevent the risk of explosion).  The light is channelled to the required area via a flexible fibre optic light guide, made from tightly bundled glass fibres. This concentrates the light and allows the light to be easily directed to where it is required.



Compiled and edited by John Sandham IEng MIET MIHEEM


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