Incandescent Light
Anything that gives out light is called a light source. There two types of sources of light:
(1) Natural
(2) Artificial.
Our main source of Natural light is the sun. The sun is a star that is a huge ball of gas. Explosions at the centre of the sun produce large amounts of energy. This energy is released as light and heat. Some of this light reaches Earth and gives us daylight. The light that comes from the sun is known as white light.
Other forms of natural light include the moon and the stars. The stars provide only small amounts of light at night as they are billions of kilometres away from Earth. A full moon, however, can provide quite a lot of light. The light from the moon is just light reflected from the sun.
A source of Artificial light can be classified as given below:
Incandescence or thermal sources: They contain a thin metal filament made out of tungsten (a type of metal). This filament becomes very hot when electricity flows through it and glows yellow–white. These bulbs last only about 1000 hours because the filament becomes thinner and thinner as it burns. These sources contain light of all wavelengths in the visible region.
Phosphorescence
The glow in dark paints made from non–toxic, non–radioactive pigments, store light energy, while they are exposed to light during the day & release that energy slowly during the night. This phosphorescent effect will be stronger if these paintings are placed in a dark place (or) places with a spot light.
Fluorescence as in a gas discharge tube: Fluorescent tubes are widely used in the office and the home. Fluorescent tubes are glass tubes that contain mercury vapor. When an electric current is passed through the mercury vapor it gives off ultraviolet light. This ultraviolet light is absorbed by phosphor powder that coats the inside of the tube and starts to glow to make a blue–white light. These lights need special electronic starters to produce the high voltage needed to start the light. This source contains light of few wavelengths.
Phosphorescence: Unlike fluorescence, a phosphorescent material does not immediately re–emit the radiation it absorbs. This source contains partly visible light and partly ultraviolet. In this, light is emitted by the principle of luminescence, which is the phenomenon of emission of light by a substance, by absorbing suitable electromagnetic radiation.