Light is electromagnetic radiation in the frequency range that is visible to the human eye. The invisible infrared, at a slightly lower frequency, and ultraviolet, at a slightly higher frequency, are sometimes erroneously referred to as infrared light and ultraviolet light. Light of a certain frequency has a wavelength that depends on the medium. Usually the wave spring is given in vacuum or air, because that is where most applications take place. The visible spectrum spans wavelengths from about 380 nm (nanometers) (violet) to 780 nm (red) in vacuum and air. In a vacuum, like all electromagnetic radiation, light travels at the speed of light. Light quanta, which play a role in quantum optics, are called photons.
The three variables that describe light are luminosity (or amplitude), color (either frequency or wavelength) and polarization, or vibration direction, which is always perpendicular to the direction of propagation. The field of research on light and the interaction of light with matter is called optics.
Particle or wave
In the 17th century, Christiaan Huygens was the first to claim that light is a wave phenomenon. This was supported by the phenomena observed in light, interference and bending. This was contradicted by Isaac Newton, who argued that light consists of a stream of fast particles. This sparked a fierce debate, which was initially settled in favor of particle theory. In the 19th century it became clear that light is an electromagnetic wave phenomenon within a specific wavelength range, thanks to the experimental work of Thomas Young, Augustin Jean Fresnel and Heinrich Hertz and the theoretical work of Lorentz. The behavior of light could be explained by solving the Maxwell equations that form the basis for all electromagnetic phenomena. With the advent (early 20th century) of quantum mechanics, a particle character of light was nevertheless established. This culminated in the development of quantum electrodynamics, which describes and predicts all interactions between charged particles with the exchange of photons completely and with great accuracy. It is a synthesis of the relativistic version of the Maxwell equations with quantum mechanics.
Origin of light
If atoms are heated enough, or otherwise become excited, the outermost electrons can reach a higher or excited energy level. When an electron returns to a lower energy level, the released energy is emitted in the form of a photon. This energy of a photon determines the frequency and thus the wavelength and thus the color of the light.
Speed of light and direction of propagation
The speed of light in an isotropic medium is equal in all directions of propagation.
Light travels in vacuum at a speed of, by definition, 299 792 458 meters per second. In a medium such as water, air or glass, the speed is lower. This is due to the interaction between the electrical vector of the light waves and the electron clouds around the atoms that make up the medium. The ratio between the speed of light in vacuum and the speed of light in a medium is the refractive index of that medium.
In special relativity it is postulated that the speed of light in vacuum is a physical constant, which does not even depend, as was assumed before the formulation of this theory in 1905, on the state of motion of the observer with respect to the light source.
When light moves through a transparent medium (such as air, water, or glass), propagation is slowed relative to vacuum by a factor called refractive index. The refractive index is defined as the ratio of the speed of light in vacuum to that in the medium: