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Thermodynamics

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Ultra Violet Spectrum

The spectrum that covers the wavelengths from 400 Å to 100 Å is called the ultra-violet spectrum. Sun is a natural source of light rich in ultra-violet radiations. An electrical arc of carbon, iron or other materials, mercury vapour lamps, discharge of electricity through hydrogen contained in quartz tubes are some of the artificial sources that give ultra-violet radiations.

Ordinary glass absorbs the ultra-violet radiations. Hence quartlenses and prisms are used. But quartz is a doubly refracting material. If a single prism of quartz is used, due to the property of double refraction of the prism, two images of the slit (ordinary and extraordinary) corresponding to a single wavelength are observed. This will reduce the sharpness of each image. To compensate for this, the collimating lens is made from right-handed quartz and the telescope objective is made from left-handed quartz. Similarly the prism used for the dispersion of the incident beam consists of two halves. The two halves are held together by glycerine. One-half is made from right-handed quartz.

In fig. ABD is a right angled prism of right-handed quartz and ADC is of left-handed quartz. For recording an ultra-violet spectrum for wavelengths shorter than 1200 Å, a concave reflection grating is used. The diffracted beam is photographed. The grating and the photographic plate are enclosed in a metal chamber which is evacuated.

Ultra-violet radiations have a variety of applications. Sterilization of rooms in which blood plasma, drugs, vaccines etc. are prepared and sealed in the containers, is done by ultra-violet radiations. Drugs, poisons, dyes etc. fluoresce under the action of ultra-violet rays. The resolving power of a microscope is increased when ultra-violet light is used for illumination. Fluorescent tubes depend on the principle of fluorescence effected by ultra-violet radiations.