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Light is the electromagnetic radiation in a specific part of the electromagnetic spectrum. The term usually refers to visible light, which is the visible spectrum of the human eye. It is responsible for vision. Visible light is usually defined as wavelength between 400-700 nm or 4.00? 10? 7 to 7.00? Between infrared (longer wavelength) and ultraviolet (shorter wavelength) is 10? 7 m。 This wavelength means a frequency range of about 430-750 terahertz (THz).

The main light source on earth is the sun. Sunlight provides the energy that green plants use to produce sugars, which are mainly produced in the form of starch, which releases energy to the organisms that digest them. The process of photosynthesis actually provides all the energy that organisms consume. Historically, another important light source for human beings is fire, from ancient bonfires to modern kerosene lamps. With the development of electric light and power system, electric lighting has effectively replaced fire light. Some animals produce their own light, a process called bioluminescence. For example, fireflies use light to find mates, while vampire squid uses light to avoid prey.

The main characteristics of visible light are intensity, propagation direction, frequency or wavelength spectrum and polarization. The speed in vacuum (299792458 meters per second) is one of the basic constants in nature. Like all types of electromagnetic radiation (EMR), visible light always moves at that speed in a vacuum.

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 EM radiation, visible light travels as waves. However, the energy given by the wave is absorbed at a single location in the way the particles are absorbed. The energy absorbed by electromagnetic wave is called photon, which represents the quantum of light. When light waves are converted and absorbed into photons, the energy of the wave immediately collapses to a single position, which is the position where photons "arrive". This is called wave function collapse. The dual properties of wave and particle of light are called wave particle duality. The study of light is called optics, which is an important research field in modern physics.

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