Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye. The primary properties of visible light are intensity, propagation-direction, frequency or wavelength spectrum and polarization.
Its speed in a vacuum, 299 792 458 metres a second (m/s), is one of the fundamental constants of nature, as with all types of electromagnetic radiation (EMR), light is found in experimental conditions to always move at this speed in a vacuum.
The main source of light on Earth is the Sun.
Light consists of dual nature which means sometimes it behaves like a particle (known as photon), which explains how the light travels in straight lines. Sometimes light behaves as the wave, which explains how light bends (or diffract) around an object.
Light as a Wave
Electromagnetic waves, including visible light, are made up of oscillating electric and magnetic fields as shown. Christian Huygens, who was a contemporary of Isaac Newton, suggested that light travels in waves. Isaac Newton, however, thought that light was compsed of particles that were too small to detect individually. In 1801 a physicist in England, Thomas Young, performed an experiment that showed that light behaves as a wave. He passed a beam of light through two thin, parallel slits. Alternating bright and dark bands appeared on a white screen some distance from the slit. Young reasoned that if light were made of particles as Newton suggested, only two bright bands of light would be projected on the white surface. The bright and dark bands demonstrated that the slits were causing light waves to interfere with each other. Sometimes this interference is constructive, and the light waves add together to create a bright patch. Sometimes the intereference is destructive and results in the light waves cancelling each other out creating dark patches on the screen.
In 1845, Michael Faraday discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric, an effect now known as Faraday rotation. This was the first evidence that light was related to electromagnetism. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.
Light as a Particle
Pierre Gassendi (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.
Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century.
The fact that light could be polarized was for the first time qualitatively explained by Newton using the particle theory.
Light is composed of photons, so we could ask if the photon has mass. The answer is then definitely "no": the photon is a massless particle. According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons, it was known that light carries momentum and will exert pressure on a surface. This is not evidence that it has mass since momentum can exist without mass.
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