What is light and how does it propagate?  

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Light is electromagnetic radiation. Generally, we think of light as the visible light (something that we can see, within the wavelength range of 400 nm to 700 nm), however there is much more to electromagnetic spectrum than visible light (such as x-rays, gamma rays, radio waves, infrared, etc.). We can also think of light as energy, for example, sunlight warms the Earth, sun bathing increases the temperature of our skin, etc. Light travels at high speeds and can travel at almost 3,00,000 km per second, through vacuum (this speed is a constant, denoted by c).

Light has dual nature and can be considered as either a particle or a wave. Light moves or propagates as a photon (particle nature) or as a self-propagating electromagnetic wave.  

Hope this helps. 

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