Sources Of Light

There are two general sources of light viz.natural source and artificial source.

Natural Sources : Our most important natural source of light is the sun. Nearly all the natural sources of light we receive comes from the sun; moonlight is sunlight reflected from the surface of the moon. Distant stars provide an extremely small amount of light.

Artificial Sources : There are several ways of producing artificial light. In general, artificial light source can be divided into three categories viz.

Thermal Sources : Example of thermal source are incandescent lamp, burning candle, etc. When object is heated until it glows or becomes incandescent, it emits all visible wavelengths along with large quantity of infrared radiation. Hence, as producers of visible radiation (i.e. luminous energy), they have a low efficiency. Generally, the efficiency of such light sources improves as the operating temperature is increased.

Gas Discharge Sources : Example of gas discharge source are neon lamp, sodium lamps, etc. In this case, light is obtained by maintaining electric current in a gas at low pressure. Such a source emits only a few wavelength. The color and intensity of light of light depends upon the nature of gas or vapor only. It may be noted that in case of light emitted by a thermal source, the spectrum is continuous. However, when light is obtained from a gaseous discharge, the spectrum is discontinuous i.e. it consists of one or more colored lines. For examples, in the case of sodium lamp, the spectrum consists mainly of two yellow lines very close together with wavelengths of 5890. These wavelengths are so close to each other that light from a sodium lamp is said to be monochromatic i.e. a light having only one wavelength.

Luminescent Sources : The familiar example of such a source is the fluorescent tube. A fluorescent tube consist of a thin-walled glass tube with fluorescent substance coated on the inside of the tube. An electric current is maintained in mercury vapors at low pressure. It emits visible radiation as well as ultraviolet radiations (invisible). The fluorescent material absorbs ultraviolet radiation and re-emits them at longer wavelengths of the visible spectrum.