Further Research
TYPICAL WAVELENGTH 5 x 10-7 m (one two-millionth of a metre) i.e. 20,000 waves per cm
SOURCES - Hot objects: Major source on Earth is the sun. Other sources range from sparks to heated metal and filaments in lamps
HAZARD DETECTION - The eye, photographic film
USES - Many
BACKGROUND
Wavelength of visible light
The region of the electromagnetic spectrum detected by the human eye and interpreted by the brain is between 10-6 and 10-7 metres.
Uses of visible light
Two applications which use visible light have helped the so-called communications revolution.
1. Optical fibres
This is usually as thin as a human hair, and has a very narrow cover of glass at extremely high transparency. It is coated with a different glass of slightly lower refractive index. Light rays passed down the core are totally internally reflected as the edges of the core behave as mirrors. This means that none of the light is lost, so the rays can travel very long distances, sometimes as long as 20km. These fibres are also replacing copper wires in the telephone system. The sound signals are converted into light, which may be that generated by a laser, which is then sent down the optical fibre. More than 10,000 conversations can be carried by a single fibre at the same time. This method is also used to end information between computers and carry many different TV channels through a single cable.
2. Lasers
A laser is a device which produces a very intense beam of light which has a fixed wavelength in which all the waves are in phase. In ordinary light there is a spread of wavelengths and the waves are not in phase. The laser concentrates and amplifies the light to give a concentrated beam of energy which can melt metals and drill holes in them. Laser beams can be used to make very accurate measurements of distance, perform delicate surgical procedures involving the eye and are used in compact disc players to convert the patterns on the disc into sound.
Medicine has benefited considerably from using lasers. They have replaced many conventional techniques since they have a number of advantages over older methods. These include shorter periods of treatment, reduced blood loss due to the laser's ability to seal blood vessels, less post-op infection and the fact that difficult, often inaccessible regions of the body can be reached more easily. In dentistry the patients suffer less pain since drills are not used. Treatments carried out include the hardening of fillings, cutting away growths on the gums and the repair and reshaping of damaged teeth.
Background on visible light
Early 17th-century investigations were designed to find out the nature of the visible spectrum. Isaac Newton produced dispersion into the colours of the visible spectrum, but the main concern was whether it consisted of a stream of particles emitted from a luminous source, or a system of waves spreading out from the source.
At the beginning of the 19th century, Thomas Young demonstrated that light suffered interference - a property of waves - just as sound waves do. He showed that a double source of light gave an interference-generated fringe pattern of alternate bright and dark bands. Different colours produced different spacings between the bands and from these he was able to calculate the wavelengths which correspond to the different colours. These were the first estimates of wavelength limits of the visible spectrum. The range of the visible region cannot be precisely fixed. This depends on the characteristics of each individual's eyes, and eyes differ.
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