Light

Light usually refers to visible light, which is electromagnetic radiation that is visible to the human eye and is responsible for the sense of sight. 

The position of visible of light in the electromagnetic spectrum. image: wikipedia


Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths.

The main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. 

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A bonfire produces light via a exothermic chemical reaction. image:R.Conan-Davies

Historically, another important source of light for humans has been fire (combustion), from ancient campfires to modern kerosene lamps. With the discovery of electricity, electric lighting technology has all but replaced firelight. 

Bioluminescence

Some species of animals generate their own light, called bioluminescence. For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.

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A bioluminescence zooplankton. image: shane Anderson/ wikipedia


Primary properties of visible light are :

  • intensity, 
  • propagation direction, 
  • frequency or wavelength spectrum, and 
  • polarisation, 


Its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum.

Physics has a broader definition of light

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.  In this sense, gamma rays, X-rays, microwaves and radio waves are also light. 

Like all types of light, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. 

The study of light, known as optics, is an important research area in modern physics.

The year 2015 was designated the International Year of Light


Source based on : Light. (2014, December 29). In Wikipedia, The Free Encyclopedia. Retrieved 07:05, January 1, 2015, from http://en.wikipedia.org/w/index.php?title=Light&oldid=640131786