Sources of Light
are two general sources of light viz.natural source and artificial
(I) Natural Sources : Our most important natural source
of light is the sun. Nearly all the natural 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.
(II) Artificial Sources : There are several ways of
producing artificial light. In general, artificial light source
can be divided into three categories viz.
(i) 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
(ii) 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
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 5890Å. 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.
(iii) 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
material absorbs ultraviolet radiation and re-emits them
at longer wavelengths of the visible spectrum.