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Thermodynamics

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Sources of Light

Light sources usually employed in the experiments on light can be mainly classified as (i) thermal sources and (ii) sources that depend on the discharge of electricity through gases. Light from the sun, flames, arcs at atmospheric pressure, tungsten filament lamps etc. belong to the first class. Mercury are lamp, discharge tubes containing gases at low pressure and high voltage sparks belong to the second category. To study the spectra of different substances (solids, liquids or gases) different sources of light are used. Some of them are described below.

Incandescent solids: Any solid heated to a high temperature emits light. As the temperature of the solid is gradually increased, the spectrum spreads from red to the violet end. Until recently, solids kept at high temperatures provided all practical sources of light. The electric glow lamp is a common example of a heated solid.