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Belongs to subject Optics

Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties. Most optical phenomena can be accounted for using the classical electromagnetic description of light. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties. When considering light's particle-like properties, the light is modelled as a collection of particles called "photons". In the 13th century in medieval Europe, English bishop Robert Grosseteste wrote on a wide range of scientific topics, and discussed light from four different perspectives: an epistemology of light, a metaphysics or cosmogony of light, an etiology or physics of light, and a theology of light, basing it on the works Aristotle and Platonism. Newtonian optics was generally accepted until the early 19th century when Thomas Young and Augustin-Jean Fresnel conducted experiments on the interference of light that firmly established light's wave nature. This work led to a theory of diffraction for light and opened an entire area of study in physical optics. Classical optics is divided into two main branches: geometrical (or ray) optics and physical (or wave) optics. In geometrical optics, light is considered to travel in straight lines, while in physical optics, light is considered as an electromagnetic wave. Specular reflection describes the gloss of surfaces such as mirrors, which reflect light in a simple, predictable way. Corner reflectors retroreflect light, producing reflected rays that travel back in the direction from which the incident rays came. Refraction occurs when light travels through an area of space that has a changing index of refraction; this principle allows for lenses and the focusing of light. For example, the propagation of light through a prism results in the light ray being deflected depending on the shape and orientation of the prism. In most materials, the index of refraction varies with the frequency of the light. In this case, no transmission occurs; all the light is reflected. As light travels down an optical fibre, it undergoes total internal reflection allowing for essentially no light to be lost over the length of the cable.

A device which produces converging or diverging light rays due to refraction is known as a lens. Lenses are characterized by their focal length: a converging lens has positive focal length, while a diverging lens has negative focal length. In physical optics, light is considered to propagate as a wave. The wavelength of visible light waves varies between 400 and 700 nm, but the term "light" is also often applied to infrared (Light waves are now generally treated as electromagnetic waves except when quantum mechanical effects have to be considered.

A vector model must also be used to model polarised light. combined waveform

wave 1

wave 2

Two waves in phase

Diffraction is the process by which light interference is most commonly observed. The simplest physical models of diffraction use equations that describe the angular separation of light and dark fringes due to light of a particular wavelength (λ). =Diffraction effects limit the ability for an optical detector to optically resolve separate light sources. Thus, blue light, with its higher refractive index, is bent more strongly than red light, resulting in the well-known rainbow pattern.

This causes the spectrum coming out of a prism to appear with red light the least refracted and blue/violet light the most refracted. In the leftmost figure above, the x and y components of the light wave are in phase. {\displaystyle When light reflects from a thin film on a surface, interference between the reflections from the film's surfaces can produce polarization in the reflected and transmitted light.

Light reflected by shiny transparent materials is partly or fully polarised, except when the light is normal (perpendicular) to the surface. Polarization occurs when light is scattered in the atmosphere. The scattered light produces the brightness and colour in clear skies. These areas of optical science typically relate to the electromagnetic or quantum properties of light but do include other topics. A major subfield of modern optics, quantum optics, deals with specifically quantum mechanical properties of light. Specialty areas of optics research include the study of how light interacts with specific materials as in crystal optics and metamaterials. Fibre-optic communication relies on lasers to transmit large amounts of information at the speed of light. Light can be used to position matter using various phenomena (see optical tweezers).

The light then passes through the lens, which focuses the light further and allows adjustment of focus. Long focus lens: angle of view narrower than a normal lens. Other results from physical and geometrical optics apply to camera optics. Rainbows are the result of a combination of internal reflection and dispersive refraction of light in raindrops.

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