Light is defined as a form of energy which when falls on the objects, enables us to see them. Light is one of the most important forms of energy. Without light, life on earth would not exist. The branch of physics, which deals with the phenomena accompanying light and vision is called optics. Important sources of light are the following-
Natural Sources of Light:
Sun is the main and brightest natural source of light. It illuminates all the planets, their satellites and other celestial bodies. Stars are also natural sources of light. Being very far away, they are not able to give sufficient light to the earth.
Another example of a natural source of light is the firefly (jugnu). It is a glowing insect. It gives very feeble light. There are certain kinds of fish too which give out light.
Artificial Sources of Light:
These are man-made sources of light. Some of the examples are candles, lanterns, gas lamps, electric bulbs and fluorescent tubes. Bodies are visible only by the light they send to our eyes and thus act as sources of light. They may be either luminous or non-luminous.
Luminous Sources- Luminous sources of light are defined as those sources of light which give out light of their own. Examples are sun, lamp, candle, electric bulb etc.
Non-Luminous Sources- These sources do not emit light of their own but reflect the light falling on them and hence become visible. The moon is not a luminous object because the moon does not produce any light of its own. It reflects the sun’s light. It acts as a cold non-luminous mirror. All the planets and moon are reflectors of light. Earth also reflects the light of the sun. The light of the sun reflected from the surface of the earth is called the earth light.
Why are some light sources hot and others cold?
Many light sources often give out heat as well. The sun gives both heat and light. A firefly gives out only light and no heat.
All solids, liquids and gases emit light when they are heated to a high temperature. The electric bulb has a wire inside, called the filament. When this filament is heated by passing an electric current through it, it gives out light. Fluorescent lights are columns of gas heated by electrical energy. In the flame of a candle, the light is given out by hot particles of carbon, heated to incandesce. The term used to describe a glowing hot object is incandesce.
Different mechanisms are responsible for the emission of light from different sources. A hot filament that glows in an electric bulb is said to be incandescent. It is a hot source of light. Here, heat is responsible for the emission of light. The burning of a candle or a match stick is another example of incandescence. A tube light is not incandescent because it does not use a filament. It is a cold source of light. A firefly gives out light but no heat, due to some chemical reactions taking place within its body. The firefly is also a cold source of light. The energy of some chemical reactions called nuclear reactions within the sun and stars is released both as heat and light.
Tubelight or fluorescent light is often called ‘cool’ light because it gives out little heat and much light. The filament of an electric bulb has a temperature of about 2000°C.