When we look at a leaf we know that it is green in colour because light bounces off the leaf to our eyes to tell us that it is green. But what is light? Which are the sources of light from which light is originated? Light is an electromagnetic radiation. A particular frequency of this radiation ( around 390-700 nm) is visible to the human eye. Everything that we see around us is because of light. Light is a form of energy and like all energies, it is produced from a source. In physics, these are called Light sources.
Types of light sources:
There are countless sources of light but they can all be categorized under either of the two following categories-
- Natural sources
- Artificial sources
Natural Light Sources:
The universe is filled with objects that emit light. Some of the light from these sources reach the earth. The following things in nature have the ability to emit light:
- The Sun is the major source of light for the earth. The sun is a massive ball of fire, at the centre of which nuclear fusion produces massive energy. This energy comes out as heat and light. The light from the sun is one of the major factors behind the sustainability of life on earth.
- Every other star produces light too, but only a small or no amount of it reaches the earth because of the huge distance.
- The moon provides light as well but it cannot produce light on its own. The light that we get from the moon is the light reflected by it from the sun.
- Some living organisms have the ability to produce light too. It is called bioluminescence. It is the effect of certain chemical reactions within the organism. Fireflies, jellyfish, glow worm, certain deep sea plants and microorganisms can be cited as examples.
- Certain other natural phenomena such as lightning and volcanic eruptions also emit light.
Artificial Light Sources:
Apart from the natural sources, light can be produced artificially too. The different light sources produced artificially can be put under three broad categories-
- Incandescent Sources:
When certain objects are heated to a high temperature, they begin to emit light. Both infrared and visible light is produced in the process.
Example- Candle, incandescent lamp.
- Luminescent Sources:
Light can be produced by accelerating charges in a luminescent material. One common way of doing it is by passing current through the material.
Example- Fluorescent tube light, electric bulb
- Gas Discharge Sources:
Passing electricity through certain gases at a very low pressure can produce light too.
Example – Neon lamp, Sodium lamp.
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