Light is a simple Electro-Magnetic radiation which is emitted by a specific body at a specific temperature. Each and very object at a given temperature, emits light, like the Sun. If it has no heat (no hydrogen), then it cannot emit light. In the same way, when a chemical reaction like nuclear fusion causes blast due to heat, large amounts of photons of lights are released.
Study of Light
Study of light and its properties is called Optics. This study is important in observation and studying natural phenomenons like Eclipses and auroras. It is also important in construction of telescopes and observing far and far away planets.
Sources of Light
Some natural sources of light are Stars and some man-made objects like bulbs, chemicals, lasers, and so on. Natural sources of light like stars emit light due to hydrogen and helium gases on their surfaces and planets receive light from stars. Earth's common source of light is the Sun (star). Almost 45 % of sun's emitted light reaches the earth's surface which is generally known as visible light and responsible for vision.
Visible and Invisible Light
Human eye has vision ranges just like human ear has audible frequencies. Light visible to human eye has wavelengths between 380 nanometres to 740 nanometres. Infrared light has high frequency and light like UV has lower frequencies. Both of these are invisible to our eyes.
Properties
Light has some common properties and some unusual. Common properties of light are it releases energy (like a particle) and it travels in a straight line and causes reflection (like a wave). Unusual property is that they behave like both particles and waves and some times behave like neither of them. The old Quantum Theory didn't define the properties of light properly, as both Huygens and Newton gave competing theories of light, saying that light is a wave and light is a particle respectively. But Michael Faraday in 1845 discovered that light became rotated when passing through an electro-magnetic field. So light was given the name Electro-Magnetic radiation and Modern Quantum Theory states that all these radiations behave like a wave in lower frequencies and like a particle in higher frequencies but don't lose completely the properties of one or another.
Speed of Light
Speed of light in vacuum is 299,792,458. But speed of light decreases in other mediums like when passing through solids or water. Light is also the fastest known moving radiation. This property of light helps the sun's light to reach earth in minutes. Also, telescopes are used to observe far away planets as light's speed is so fast we can observe movement of far away planets and stars.
Light - A Particle and a Wave
Christian Huygens first stated the Particle Theory of Light and stated that light was a particle and proved that light travels with constant velocity and can release energy like a Particle. When Isaac Newton studied Huygens' theory, he made another theory equally competing to the previous theory and stated that light was a wave as it can be reflected and it is an energy that spreads in all directions. This made the Old Modern Theory to not be very perfect.
Old and New Quantum Theories
Old Quantum theory of light was not very perfect but the modern version of it describes light very well and has proofs and explanations for it. When Huygens and Newton gave their own theories, the old theory states that light is both wave and a particle but didn't have a perfect definition. But in 1845, Michael Faraday explained that light had the properties of electro-magnetic radiations when he observed that light rotated according to the magnetic fields. This gave a perfect definition for light that it is an EM Radiation which possessed both the properties of wave and particle.
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See the following video from YouTube describing the wave particle duality of light:
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Article by - M. Santosh
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