Theory:

When we enter a completely dark room, we cannot see anything around us, but everything is visible at the moment we switch on the light. This is because light can be detected with the help of the human eye. Light is a form of energy that helps us to see things around us. Without light, we cannot see an object.
  
How does light help to make things visible?
 
When light rays fall on the object, it will get reflected, and when it enters our eye, we can see the object.
 
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Reflection of light
  
Sources of Light:
The objects which can emit light are known as light sources. There are two sources of light.
  • Natural sources of light
  • Artificial sources of light
Natural sources of light:
  • The source which emits light naturally is known as natural source of light. The Sun is the primary and the major source of natural light in our solar system.
  • Some living organisms such as fireflies, jellyfish, glow-worm, certain deep-sea plants, and some microorganisms have the ability to produce light, and this phenomenon is called as bioluminescence. It is due to the effect of certain chemical reactions occurring in the organism.
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Glow-worm
  
Artificial sources of light:
The source which emits light artificially is termed as an artificial source of light. Some of the artificial sources of light are the flame of a candle, incandescent lamp, neon lamp, Sodium lamp etc. Artificial sources of light can be classified into three categories they are
  • Incandescent Sources
  • Gas Discharge Sources
  • Luminescent sources
Incandescent Sources:
When certain objects are treated with a high temperature, they begin to emit light.
Example:
The glowing of hot iron rod and the red burners on an electric stove
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Candle
  
Luminescent sources:
Certain materials can produce light by accelerating the charges in them. One common way of doing this is bypassing current through the conducting material.
Example:
Fluorescent tube light, electric bulb
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Fluorescent tube light
  
Gas Discharge Sources:
When we pass electricity through certain gases at very low pressure, light can be produced.
Example:
Neon lamp, Sodium lamp
1200px-2015-02-14_06_37_24_Street_light_post_with_both_sodium_vapor_and_mercury_vapor_lights_in_the_Roy's_parking_lot_in_Elko,_Nevada.jpg
Sodium lamp
Reference:
https://pixabay.com/illustrations/candle-candle-light-candles-light-5172611/
https://commons.wikimedia.org/wiki/File:Close-up_view_of_a_bioluminescent_beetle_Elateroidea.jpg
https://commons.wikimedia.org/wiki/File:Earth_absorbing_the_sunlight_.._to_live.jpg
https://pixabay.com/photos/neon-tube-neon-light-hell-light-269367/
https://commons.wikimedia.org/wiki/File:2015-02-14_06_37_24_Street_light_post_with_both_sodium_vapor_and_mercury_vapor_lights_in_the_Roy%27s_parking_lot_in_Elko,_Nevada.JPG