Light

This article is about the physical and cosmological properties of **light**. Visible light is electromagnetic radiation within the electromagnetic spectrum.

Its speed in a vacuum is one of the fundamental constants of nature and all of the cosmos. Light propagates by massless elementary particles called photons.

The study of light is known as optics. Optical phenomena such as rainbows, the aurora borealis, reflection and refraction show the nature of light. *Rayleigh scattering* is the scattering of sunlight by the gas molecules and microscopic density fluctuations in the atmosphere. Blue light scatters much more than other colors, and that's why we see a blue sky. When the Sun is near the horizon, such as during sunset, more of the blue light is scattered away from an observer, leaving longer wavelength colors to such as red to show a sunset.

The main source of natural light on Earth is the Sun, and historically for humans it has been fire. With the development of electricity, electric lighting has effectively replaced firelight. Lasers (and associated weapons) are built using light amplification and radiation methods.

“What is light?” is a question that has intrigued scientists for centuries. In 1675 Isaac Newton proposed that light was a stream of tiny particles. However, Christiaan Huygens suggested that light consisted of waves, but Newton was more prestigious at the time and his theory was more widely accepted.

Around 1800, Thomas Young began a series of experiments that provided support for Huygens’ wave theory. In a modern version of Young’s experiment (showing also the uncertainty principle), a laser equally illuminates two parallel slits in an opaque surface. The pattern that the light makes as it passes through the two slits is observed on a distant screen. Young used geometrical arguments to show that the superposition of light waves from the two slits explains the observed series of equally spaced bands (fringes) of light and dark regions, representing constructive and destructive interference. This is similar to the tossing of two stones into a lake and watching the waves running into one another and sometimes canceling each other out or building up to form even larger waves.

If we carry out the same experiment with a beam of electrons instead of light, the resulting interference pattern is similar. If the electrons behaved only as particles, one might expect to simply see two bright spots corresponding to the two slits. When single electrons are sent through the slits one at a time, an interference pattern is produced that is similar to that produced for waves passing through both holes at once. This behavior applies to all subatomic particles, not just photons and electrons, and suggests that light and other subatomic particles have a mysterious combination of particle and wavelike behavior. This exhibits the *duality* of light.

In 1845, Michael Faraday discovered linearly polarized light rotated when the light rays travel along a magnetic field direction. This was the first evidence that light was related to electromagnetism. James Clerk Maxwell then discovered that self-propagating electromagnetic waves would travel through space at the speed of light. From this, he concluded that light was a form of electromagnetic radiation, which he published in *A Treatise on Electricity and Magnetism* in 1873.

In 1900 Max Planck suggested that light waves could gain or lose energy only in finite amounts related to their frequency, and called these particles of light energy "quanta". In 1905, Albert Einstein used the idea of light quanta to explain the photoelectric effect. In 1926 Gilbert Lewis named these light quanta particles photons. This led to the modern theory of quantum mechanics which pictures light as as *both* a particle and a wave.

The mass equivalent of the energy of a photon at the peak of the spectrum of the cosmic microwave background radiation is 4.2×10−40 kg, which is second lightest particle in nature behind the graviton.

So, from the origins of the particle theory and wave theory, the double-slit and other experiments led to the electromagnetic theory and quantum theory. From these theories the properties of light are deduced:

* intensity (electromagnetic)
* polarization (particle)
* frequency (wave)
* propagation in a direction via photons (quantum)

In 2018 a new form of light involving polaritons was discovered, that could be useful in the development of quantum computers.

According to Einstein's general theory of relativity, an object passing in front of another bright object would bend the light of the more distant object and cause it to appear to move from its original position. Observation of light approaching a black hole shows that it bends, and is then swallowed as it passes the event horizon.

With the old laws of physics the speed of light is a constant, but with the new laws of physics faster than light travel is possible.