Content deleted Content added
m →Classical optics: sentence case |
m Open access bot: doi updated in citation with #oabot. |
||
| (14 intermediate revisions by 7 users not shown) | |||
Line 1:
{{Short description|Branch of physics that studies light}}
{{About|the branch of physics|the book by Sir Isaac Newton|Opticks{{!}}''Opticks''|other uses|Optic (disambiguation)}}
[[File:ANDY7187.jpg|thumb|A researcher working on an optical system|300x300px]]
{{TopicTOC-Physics}}
Line 16:
{{See also|Timeline of electromagnetism and classical optics}}
[[File:Nimrud lens British Museum.jpg|thumb|right|The Nimrud lens]]
Optics began with the development of lenses by the [[ancient Egypt]]ians and [[Mesopotamia]]ns. The earliest known lenses, made from polished [[crystal]], often [[quartz]], date from as early as 2000 BC from [[Crete]] (Archaeological Museum of Heraclion, Greece). Lenses from [[Rhodes]] date around 700 BC, as do [[Assyria]]n lenses such as the [[Nimrud lens]].<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/380186.stm |title=World's oldest telescope? |work=BBC News |date=July 1, 1999 |access-date=Jan 3, 2010 |url-status=live |archive-url=https://web.archive.org/web/20090201185740/http://news.bbc.co.uk/1/hi/sci/tech/380186.stm |archive-date=February 1, 2009 }}</ref> The [[ancient Roman]]s and [[Ancient Greece|Greeks]] filled glass spheres with water to make lenses. These practical developments were followed by the development of theories of light and vision by ancient [[Greek philosophy|Greek]] and [[Indian philosophy|Indian]] philosophers, and the development of [[geometrical optics]] in the [[Greco-Roman world]]. The word ''optics'' comes from the [[ancient Greek]] word {{lang|grc|ὀπτική}},
Greek philosophy on optics broke down into two opposing theories on how vision worked, the [[intromission theory]] and the [[emission theory (vision)|emission theory]].<ref>[http://www.stanford.edu/class/history13/earlysciencelab/body/eyespages/eye.html A History Of The Eye] {{webarchive|url=https://web.archive.org/web/20120120085632/http://www.stanford.edu/class/history13/earlysciencelab/body/eyespages/eye.html |date=2012-01-20 }}. stanford.edu. Retrieved 2012-06-10.</ref> The intromission approach saw vision as coming from objects casting off copies of themselves (called eidola) that were captured by the eye. With many propagators including [[Democritus]], [[Epicurus]], [[Aristotle]] and their followers, this theory seems to have some contact with modern theories of what vision really is, but it remained only speculation lacking any experimental foundation.
Line 23:
[[Ptolemy]], in his treatise ''[[Ptolemy#Optics|Optics]]'', held an extramission-intromission theory of vision: the rays (or flux) from the eye formed a cone, the vertex being within the eye, and the base defining the visual field. The rays were sensitive, and conveyed information back to the observer's intellect about the distance and orientation of surfaces. He summarized much of Euclid and went on to describe a way to measure the [[angle of refraction]], though he failed to notice the empirical relationship between it and the angle of incidence.<ref name=Ptolemy>{{cite book |title=Ptolemy's theory of visual perception: an English translation of the Optics with introduction and commentary |author=Ptolemy |editor=A. Mark Smith |publisher=DIANE Publishing |year=1996 |isbn=978-0-87169-862-9}}</ref> [[Plutarch]] (1st–2nd century AD) described multiple reflections on spherical mirrors and discussed the creation of magnified and reduced images, both real and imaginary, including the case of [[chirality]] of the images.
[[File:Ibn Sahl manuscript.jpg|thumb|right|upright|Reproduction of a page of [[Ibn Sahl (mathematician)|Ibn Sahl]]'s manuscript showing his knowledge of [[Snell's law|the law of refraction]] ]]
During the [[Middle Ages]], Greek ideas about optics were resurrected and extended by writers in the [[Muslim world]]. One of the earliest of these was [[Al-Kindi]] ({{Circa|801}}–873) who wrote on the merits of Aristotelian and Euclidean ideas of optics, favouring the emission theory since it could better quantify optical phenomena.<ref>Adamson, Peter (2006). "Al-Kindi¯ and the reception of Greek philosophy". In Adamson, Peter; Taylor, R.. The Cambridge companion to Arabic philosophy. Cambridge University Press. p. 45. {{ISBN|978-0-521-52069-0}}.</ref> In 984, the [[Persia]]n mathematician [[Ibn Sahl (mathematician)|Ibn Sahl]] wrote the treatise "On burning mirrors and lenses", correctly describing a law of refraction equivalent to Snell's law.<ref name=j1>{{cite journal |doi=10.1086/355456 |last=Rashed |first=Roshdi |title=A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses |journal=Isis |volume=81 |issue = 3 |year=1990 |pages=464–491 |jstor=233423|s2cid=144361526 }}</ref> He used this law to compute optimum shapes for lenses and [[curved mirror]]s. In the early 11th century, Alhazen (Ibn al-Haytham) wrote the ''[[Book of Optics]]'' (''Kitab al-manazir'') in which he explored reflection and refraction and proposed a new system for explaining vision and light based on observation and experiment.<ref>{{multiref2 | {{cite book |editor1-last=Hogendijk |editor1-first=Jan P. |editor2-last=Sabra |editor2-first=Abdelhamid I. |year=2003|title=The Enterprise of Science in Islam: New Perspectives |pages=85–118 |publisher=MIT Press |isbn=978-0-262-19482-2 |oclc=50252039}}
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,<ref>D.C. Lindberg, ''Theories of Vision from al-Kindi to Kepler'', (Chicago: Univ. of Chicago Pr., 1976), pp. 94–99.</ref> basing it on the works of Aristotle and Platonism. Grosseteste's most famous disciple, [[Roger Bacon]], wrote works citing a wide range of recently translated optical and philosophical works, including those of Alhazen, Aristotle, [[Avicenna]], [[Averroes]], Euclid, al-Kindi, Ptolemy, Tideus, and [[Constantine the African]]. Bacon was able to use parts of glass spheres as [[magnifying glass]]es to demonstrate that light reflects from objects rather than being released from them.
The first wearable eyeglasses were invented in Italy around 1286.<ref>{{cite book |
This was the start of the optical industry of grinding and polishing lenses for these "spectacles", first in Venice and Florence in the thirteenth century,<ref>[http://galileo.rice.edu/sci/instruments/telescope.html "The Galileo Project > Science > The Telescope" by Al Van Helden] {{webarchive|url=https://web.archive.org/web/20120320091537/http://galileo.rice.edu/sci/instruments/telescope.html |date=2012-03-20 }}. Galileo.rice.edu. Retrieved 2012-06-10.</ref> and later in the spectacle making centres in both the Netherlands and Germany.<ref>{{cite book |author=Henry C. King |title=The History of the Telescope |url=https://books.google.com/books?id=KAWwzHlDVksC&pg=PR1 |year=2003 |publisher=Courier Dover Publications |isbn=978-0-486-43265-6 |page=27 |url-status=live |archive-url=https://web.archive.org/web/20160617095507/https://books.google.com/books?id=KAWwzHlDVksC&pg=PR1 |archive-date=2016-06-17 }}</ref> Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses rather than using the rudimentary optical theory of the day (theory which for the most part could not even adequately explain how spectacles worked).<ref>{{cite book |author1=Paul S. Agutter |author2=Denys N. Wheatley |title=Thinking about Life: The History and Philosophy of Biology and Other Sciences |url=https://books.google.com/books?id=Gm4bqeBMR8cC&pg=PA17 |year=2008 |publisher=Springer |isbn=978-1-4020-8865-0 |page=17 |url-status=live |archive-url=https://web.archive.org/web/20160516134901/https://books.google.com/books?id=Gm4bqeBMR8cC&pg=PA17 |archive-date=2016-05-16 }}</
[[File:Kepler - Ad Vitellionem paralipomena quibus astronomiae pars optica traditur, 1604 - 158093 F.jpg|thumb|left|The first treatise about optics by [[Johannes Kepler]],
[[File:Opticks.jpg|thumb|right|upright|Cover of the first edition of Newton's ''Opticks'' (1704)]]
[[File:Table of Opticks, Cyclopaedia, Volume 2.jpg|thumb|upright|Board with optical devices, 1728 Cyclopaedia]]
In the early 17th century, [[Johannes Kepler]] expanded on geometric optics in his writings, covering lenses, reflection by flat and curved mirrors, the principles of [[pinhole camera]]s, inverse-square law governing the intensity of light, and the optical explanations of astronomical phenomena such as [[Lunar eclipse|lunar]] and [[solar eclipse]]s and astronomical [[parallax]]. He was also able to correctly deduce the role of the [[retina]] as the actual organ that recorded images, finally being able to scientifically quantify the effects of different types of lenses that spectacle makers had been observing over the previous 300 years.
Optical theory progressed in the mid-17th century with [[The World (Descartes)#Cartesian theory on light|treatises]] written by philosopher [[René Descartes]], which explained a variety of optical phenomena including reflection and refraction by assuming that light was emitted by objects which produced it.<ref name=Sabra>{{cite book|title=Theories of light, from Descartes to Newton|author=A.I. Sabra|publisher=CUP Archive|year=1981|isbn=978-0-521-28436-3}}</ref> This differed substantively from the ancient Greek emission theory. In the late 1660s and early 1670s, [[Isaac Newton]] expanded Descartes's ideas into a [[corpuscle theory of light]], famously determining that white light was a mix of colours that can be separated into its component parts with a [[Prism (optics)|prism]]. In 1690, [[Christiaan Huygens]] proposed a wave theory for light based on suggestions that had been made by [[Robert Hooke]] in 1664. Hooke himself publicly criticised Newton's theories of light and the feud between the two lasted until Hooke's death. In 1704, Newton published ''[[Opticks]]'' and, at the time, partly because of his success in other areas of physics, he was generally considered to be the victor in the debate over the nature of light.<ref name=Sabra />
Line 55:
When a ray of light hits the boundary between two transparent materials, it is divided into a reflected and a refracted ray.
* The law of refraction says that the refracted ray lies in the plane of incidence, and the sine of the angle of incidence divided by the sine of the angle of refraction is a constant: <math display="block">\frac {\sin {\theta_1}}{\sin {\theta_2}} = n,</math> where {{math|''n''}} is a constant for any two materials and a given colour of light. If the first material is air or vacuum, {{math|''n''}} is the [[refractive index]] of the second material.▼
▲where {{math|''n''}} is a constant for any two materials and a given colour of light. If the first material is air or vacuum, {{math|''n''}} is the [[refractive index]] of the second material.
The laws of reflection and refraction can be derived from [[Fermat's principle]] which states that ''the path taken between two points by a ray of light is the path that can be traversed in the least time.''<ref>{{cite book|author=Arthur Schuster|title=An Introduction to the Theory of Optics|url=https://archive.org/details/anintroductiont02schugoog|year=1904|publisher=E. Arnold|page=[https://archive.org/details/anintroductiont02schugoog/page/n62 41]}}</ref>
Line 73 ⟶ 68:
Reflections can be divided into two types: [[specular reflection]] and [[diffuse reflection]]. Specular reflection describes the gloss of surfaces such as mirrors, which reflect light in a simple, predictable way. This allows for the production of reflected images that can be associated with an actual ([[real image|real]]) or extrapolated ([[virtual image|virtual]]) location in space. Diffuse reflection describes non-glossy materials, such as paper or rock. The reflections from these surfaces can only be described statistically, with the exact distribution of the reflected light depending on the microscopic structure of the material. Many diffuse reflectors are described or can be approximated by [[Lambert's cosine law]], which describes surfaces that have equal [[luminance]] when viewed from any angle. Glossy surfaces can give both specular and diffuse reflection.
In specular reflection, the direction of the reflected ray is determined by the angle the incident ray makes with the [[surface normal]], a line perpendicular to the surface at the point where the ray hits. The incident and reflected rays and the normal lie in a single plane, and the angle between the reflected ray and the surface normal is the same as that between the incident ray and the normal.
For [[Plane mirror|flat mirrors]], the law of reflection implies that images of objects are upright and the same distance behind the mirror as the objects are in front of the mirror. The image size is the same as the object size. The law also implies that [[mirror image]]s are parity inverted, which we perceive as a left-right inversion. Images formed from reflection in two (or any even number of) mirrors are not parity inverted. [[Corner reflector]]s produce reflected rays that travel back in the direction from which the incident rays came.
Mirrors with curved surfaces can be modelled by ray tracing and using the law of reflection at each point on the surface. For [[Parabolic reflector|mirrors with parabolic surfaces]], parallel rays incident on the mirror produce reflected rays that converge at a common [[focus (optics)|focus]]. Other curved surfaces may also focus light, but with aberrations due to the diverging shape causing the focus to be smeared out in space. In particular, spherical mirrors exhibit [[spherical aberration]]. Curved mirrors can form images with a magnification greater than or less than one, and the magnification can be negative, indicating that the image is inverted. An upright image formed by reflection in a mirror is always virtual, while an inverted image is real and can be projected onto a screen.
====Refractions====
{{Main|Refraction}}
[[File:Snells law.svg|thumb|upright=1.35|Illustration of Snell's Law for the case {{math|''n''<sub>1</sub> < ''n''<sub>2</sub>}}, such as air/water interface]]
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. The simplest case of refraction occurs when there is an [[Interface (chemistry)|interface]] between a uniform medium with index of refraction
where
The index of refraction of a medium is related to the speed, {{math|''v''}}, of light in that medium by
where {{math|''c''}} is the [[speed of light in vacuum]].
Snell's Law can be used to predict the deflection of light rays as they pass through linear media as long as the indexes of refraction and the geometry of the media are known. 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, known as [[dispersion (optics)|dispersion]]. Taking this into account, Snell's Law can be used to predict how a prism will disperse light into a spectrum.{{sfnp|Young|Freedman|2020|p=1116}} The discovery of this phenomenon when passing light through a prism is famously attributed to Isaac Newton.
Some media have an index of refraction which varies gradually with position and, therefore, light rays in the medium are curved. This effect is responsible for [[mirage]]s seen on hot days: a change in index of refraction air with height causes light rays to bend, creating the appearance of specular reflections in the distance (as if on the surface of a pool of water). Optical materials with varying indexes of refraction are called gradient-index (GRIN) materials. Such materials are used to make [[gradient-index optics]].<ref>{{cite book |first=E.W. |last=Marchand |title=Gradient Index Optics |location=New York |publisher=Academic Press |year=1978}}</ref>
For light rays travelling from a material with a high index of refraction to a material with a low index of refraction, Snell's law predicts that there is no
=====Lenses=====
{{main|Lens (optics)}}
[[File:lens3b.svg|upright=1.65|thumb|A ray tracing diagram for a converging lens]]
A device that 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. Smaller focal length indicates that the lens has a stronger converging or diverging effect. The focal length of a simple lens in air is given by the [[lensmaker's equation]].
Ray tracing can be used to show how images are formed by a lens. For a [[thin lens]] in air, the location of the image is given by the simple equation
where
[[File:Lens1.svg|upright=1.65|thumb]]
Incoming parallel rays are focused by a converging lens onto a spot one focal length from the lens, on the far side of the lens. This is called the rear focal point of the lens. Rays from an object at a finite distance are focused further from the lens than the focal distance; the closer the object is to the lens, the further the image is from the lens.
With diverging lenses, incoming parallel rays diverge after going through the lens, in such a way that they seem to have originated at a spot one focal length in front of the lens. This is the lens's front focal point. Rays from an object at a finite distance are associated with a virtual image that is closer to the lens than the focal point, and on the same side of the lens as the object. The closer the object is to the lens, the closer the virtual image is to the lens. As with mirrors, upright images produced by a single lens are virtual, while inverted images are real.
Lenses suffer from [[optical aberration|aberrations]] that distort images. ''Monochromatic aberrations'' occur because the geometry of the lens does not perfectly direct rays from each object point to a single point on the image, while [[chromatic aberration]] occurs because the index of refraction of the lens varies with the wavelength of the light.
[[File:Thin lens images.svg|thumb|none|upright=2.25|Images of black letters in a thin convex lens of focal length
===Physical optics===
Line 140 ⟶ 135:
{{Main|Superposition principle|Interference (optics)}}
In the absence of [[nonlinear optics|nonlinear]] effects, the superposition principle can be used to predict the shape of interacting waveforms through the simple addition of the disturbances.
{|
Line 157 ⟶ 152:
[[File:Dieselrainbow.jpg|thumb|right|upright=1.35|When oil or fuel is spilled, colourful patterns are formed by thin-film interference.]]
Since the Huygens–Fresnel principle states that every point of a wavefront is associated with the production of a new disturbance, it is possible for a wavefront to interfere with itself constructively or destructively at different locations producing bright and dark fringes in regular and predictable patterns.
The appearance of [[Thin film optics|thin films and coatings]] is directly affected by interference effects. [[Antireflective coating]]s use destructive interference to reduce the reflectivity of the surfaces they coat, and can be used to minimise glare and unwanted reflections. The simplest case is a single layer with a thickness of one-fourth the wavelength of incident light. The reflected wave from the top of the film and the reflected wave from the film/material interface are then exactly 180° out of phase, causing destructive interference. The waves are only exactly out of phase for one wavelength, which would typically be chosen to be near the centre of the visible spectrum, around 550 nm. More complex designs using multiple layers can achieve low reflectivity over a broad band, or extremely low reflectivity at a single wavelength.
Constructive interference in thin films can create a strong reflection of light in a range of wavelengths, which can be narrow or broad depending on the design of the coating. These films are used to make [[dielectric mirror]]s, [[interference filter]]s, [[heat reflector]]s, and filters for colour separation in [[colour television]] cameras. This interference effect is also what causes the colourful rainbow patterns seen in oil slicks.
====Diffraction and optical resolution====
{{Main|Diffraction|Optical resolution}}
[[File:Double slit diffraction.svg|upright=1.35|right|thumb|Diffraction on two slits separated by distance
Diffraction is the process by which light interference is most commonly observed. The effect was first described in 1665 by [[Francesco Maria Grimaldi]], who also coined the term from the Latin
The first physical optics model of diffraction that relied on the Huygens–Fresnel principle was developed in 1803 by Thomas Young in his interference experiments with the interference patterns of two closely spaced slits. Young showed that his results could only be explained if the two slits acted as two unique sources of waves rather than corpuscles.<ref>{{cite book |
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 ({{mvar|λ}}). In general, the equation takes the form
This equation is modified slightly to take into account a variety of situations such as diffraction through a single gap, diffraction through multiple slits, or diffraction through a [[diffraction grating]] that contains a large number of slits at equal spacing.
[[X-ray diffraction]] makes use of the fact that atoms in a crystal have regular spacing at distances that are on the order of one [[angstrom]]. To see diffraction patterns, x-rays with similar wavelengths to that spacing are passed through the crystal. Since crystals are three-dimensional objects rather than two-dimensional gratings, the associated diffraction pattern varies in two directions according to [[Bragg reflection]], with the associated bright spots occurring in [[Diffraction topography|unique patterns]] and
▲where <math>d</math> is the separation between two wavefront sources (in the case of Young's experiments, it was [[Double-slit experiment|two slits]]), <math>\theta</math> is the angular separation between the central fringe and the <math>m</math>th order fringe, where the central maximum is <math>m = 0</math>.<ref name=diffraction>{{cite book|title=University Physics 8e|author=H.D. Young|publisher=Addison-Wesley|year=1992|isbn=978-0-201-52981-4|url-access=registration|url=https://archive.org/details/universityphysic8edyoun}}Chapter 38</ref>
Diffraction effects limit the ability of an optical detector to [[optical resolution|optically resolve]] separate light sources. In general, light that is passing through an [[aperture]] will experience diffraction and the best images that can be created (as described in [[Diffraction-limited system|diffraction-limited optics]]) appear as a central spot with surrounding bright rings, separated by dark nulls; this pattern is known as an [[Airy pattern]], and the central bright lobe as an [[Airy disk]].{{sfnp|Hecht|2017|p=482}} The size of such a disk is given by <math display="block"> \sin \theta = 1.22 \frac{\lambda}{D}</math> where
▲This equation is modified slightly to take into account a variety of situations such as diffraction through a single gap, diffraction through multiple slits, or diffraction through a [[diffraction grating]] that contains a large number of slits at equal spacing.<ref name=diffraction /> More complicated models of diffraction require working with the mathematics of [[Fresnel diffraction|Fresnel]] or Fraunhofer diffraction.<ref name=phyoptics>{{cite book|author=R.S. Longhurst|title=Geometrical and Physical Optics, 2nd Edition|year=1968|publisher=Longmans|location=London|bibcode=1967gpo..book.....L}}</ref>
For astronomical imaging, the atmosphere prevents optimal resolution from being achieved in the visible spectrum due to the atmospheric [[scattering]] and dispersion which cause stars to [[Scintillation (astronomy)|twinkle]]. Astronomers refer to this effect as the quality of [[astronomical seeing]]. Techniques known as [[adaptive optics]] have been used to eliminate the atmospheric disruption of images and achieve results that approach the diffraction limit.<ref>{{cite thesis |
▲[[X-ray diffraction]] makes use of the fact that atoms in a crystal have regular spacing at distances that are on the order of one [[angstrom]]. To see diffraction patterns, x-rays with similar wavelengths to that spacing are passed through the crystal. Since crystals are three-dimensional objects rather than two-dimensional gratings, the associated diffraction pattern varies in two directions according to [[Bragg reflection]], with the associated bright spots occurring in [[Diffraction topography|unique patterns]] and <math>d</math> being twice the spacing between atoms.<ref name=diffraction />
▲where ''θ'' is the angular resolution, ''λ'' is the wavelength of the light, and ''D'' is the [[diameter]] of the lens aperture. If the angular separation of the two points is significantly less than the Airy disk angular radius, then the two points cannot be resolved in the image, but if their angular separation is much greater than this, distinct images of the two points are formed and they can therefore be resolved. [[John Strutt, 3rd Baron Rayleigh|Rayleigh]] defined the somewhat arbitrary "[[Rayleigh criterion]]" that two points whose angular separation is equal to the Airy disk radius (measured to first null, that is, to the first place where no light is seen) can be considered to be resolved. It can be seen that the greater the diameter of the lens or its aperture, the finer the resolution.<ref name=diffraction /> [[Astronomical interferometer|Interferometry]], with its ability to mimic extremely large baseline apertures, allows for the greatest angular resolution possible.<ref name=interferometry />
▲For astronomical imaging, the atmosphere prevents optimal resolution from being achieved in the visible spectrum due to the atmospheric [[scattering]] and dispersion which cause stars to [[Scintillation (astronomy)|twinkle]]. Astronomers refer to this effect as the quality of [[astronomical seeing]]. Techniques known as [[adaptive optics]] have been used to eliminate the atmospheric disruption of images and achieve results that approach the diffraction limit.<ref>{{cite thesis |type=PhD |last=Tubbs |first=Robert Nigel |title=Lucky Exposures: Diffraction limited astronomical imaging through the atmosphere |url=http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/ |date=September 2003 |publisher=Cambridge University |archive-url=https://web.archive.org/web/20081005013157/http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/ |archive-date=2008-10-05}}</ref>
====Dispersion and scattering====
Line 196 ⟶ 184:
Refractive processes take place in the physical optics limit, where the wavelength of light is similar to other distances, as a kind of scattering. The simplest type of scattering is [[Thomson scattering]] which occurs when electromagnetic waves are deflected by single particles. In the limit of Thomson scattering, in which the wavelike nature of light is evident, light is dispersed independent of the frequency, in contrast to [[Compton scattering]] which is frequency-dependent and strictly a [[quantum mechanical]] process, involving the nature of light as particles. In a statistical sense, elastic scattering of light by numerous particles much smaller than the wavelength of the light is a process known as [[Rayleigh scattering]] while the similar process for scattering by particles that are similar or larger in wavelength is known as [[Mie scattering]] with the [[Tyndall effect]] being a commonly observed result. A small proportion of light scattering from atoms or molecules may undergo [[Raman scattering]], wherein the frequency changes due to excitation of the atoms and molecules. [[Brillouin scattering]] occurs when the frequency of light changes due to local changes with time and movements of a dense material.<ref>{{cite book|author1=C.F. Bohren |author2=D.R. Huffman |name-list-style=amp |title=Absorption and Scattering of Light by Small Particles|publisher=Wiley|year=1983|isbn=978-0-471-29340-8}}</ref>
Dispersion occurs when different frequencies of light have different [[phase velocity|phase velocities]], due either to material properties (''material dispersion'') or to the geometry of an [[optical waveguide]] (''waveguide dispersion''). The most familiar form of dispersion is a decrease in index of refraction with increasing wavelength, which is seen in most transparent materials. This is called "normal dispersion". It occurs in all [[dielectric|dielectric materials]], in wavelength ranges where the material does not absorb light.<ref name=J286>{{cite book|author=J.D. Jackson|title=Classical Electrodynamics|edition=2nd|publisher=Wiley|year=1975|isbn=978-0-471-43132-9|page=[https://archive.org/details/classicalelectro00jack_0/page/286 286]|url=https://archive.org/details/classicalelectro00jack_0/page/286}}</ref> In wavelength ranges where a medium has significant absorption, the index of refraction can increase with wavelength. This is called "anomalous dispersion".
The separation of colours by a prism is an example of normal dispersion. At the surfaces of the prism, Snell's law predicts that light incident at an angle {{mvar|θ}} to the normal will be refracted at an angle {{math|arcsin(sin (''θ'') / ''n'')}}. Thus, blue light, with its higher refractive index, is bent more strongly than red light, resulting in the well-known [[rainbow]] pattern.
[[File:Wave group.gif|frame|Dispersion: two sinusoids propagating at different speeds make a moving interference pattern. The red dot moves with the [[phase velocity]], and the green dots propagate with the [[group velocity]]. In this case, the phase velocity is twice the group velocity. The red dot overtakes two green dots, when moving from the left to the right of the figure. In effect, the individual waves (which travel with the phase velocity) escape from the wave packet (which travels with the group velocity).]]
Material dispersion is often characterised by the [[Abbe number]], which gives a simple measure of dispersion based on the index of refraction at three specific wavelengths. Waveguide dispersion is dependent on the [[propagation constant]].
where
where
If
The result of group velocity dispersion, whether negative or positive, is ultimately temporal spreading of the pulse. This makes dispersion management extremely important in optical communications systems based on [[optical fibre]]s, since if dispersion is too high, a group of pulses representing information will each spread in time and merge, making it impossible to extract the signal.<ref name=optnet />
====Polarisation <span class="anchor" id="Polarization"></span>====
{{Main|
The typical way to consider
<div style="float:left;width:170px">
[[File:Polarisation (Linear).svg|center|Linear
{{center|''Linear''}}
</div>
<div style="float:left;width:170px">
[[File:Polarisation (Circular).svg|center|Circular
{{center|''Circular''}}
</div>
<div style="float:left;width:170px">
[[File:Polarisation (Elliptical).svg|center|Elliptical
{{center|''Elliptical
</div>
{{Clear}}
In the leftmost figure above, the {{mvar|x}} and {{mvar|y}} components of the light wave are in phase. In this case, the ratio of their strengths is constant, so the direction of the electric vector (the vector sum of these two components) is constant. Since the tip of the vector traces out a single line in the plane, this special case is called linear
In the middle figure, the two orthogonal components have the same amplitudes and are 90° out of phase. In this case, one component is zero when the other component is at maximum or minimum amplitude. There are two possible phase relationships that satisfy this requirement: the
In all other cases, where the two components either do not have the same amplitudes and/or their phase difference is neither zero nor a multiple of 90°, the
=====Changing
Media that have different indexes of refraction for different
[[File:Malus law.svg|right|thumb|upright=1.6|A polariser changing the orientation of linearly polarised light.
Media that reduce the amplitude of certain
where {{math|''I''{{sub|0}}}} is the initial intensity, and {{math|''θ''{{sub|i}}}} is the angle between the light's initial polarisation direction and the axis of the polariser.{{sfnmp |1a1=Hecht|1y=2017|1p=338 |2a1=Young|2a2=Freedman|2y=2020|2pp=1119–1121}}
A beam of unpolarised light can be thought of as containing a uniform mixture of linear
In practice, some light is lost in the polariser and the actual transmission of unpolarised light will be somewhat lower than this, around 38% for Polaroid-type polarisers but considerably higher (>49.9%) for some birefringent prism types.
In addition to birefringence and dichroism in extended media,
=====Natural light=====
[[File:CircularPolarizer.jpg|right|thumb|upright=1.8|The effects of a [[photographic filter#Polarizer|polarising filter]] on the sky in a photograph. Left picture is taken without polariser. For the right picture, filter was adjusted to eliminate certain
Most sources of electromagnetic radiation contain a large number of atoms or molecules that emit light. The orientation of the electric fields produced by these emitters may not be [[statistical correlation|correlated]], in which case the light is said to be ''unpolarised''. If there is partial correlation between the emitters, the light is ''partially polarised''. If the
Light reflected by shiny transparent materials is partly or fully polarised, except when the light is normal (perpendicular) to the surface. It was this effect that allowed the mathematician Étienne-Louis Malus to make the measurements that allowed for his development of the first mathematical models for polarised light.
==Modern optics==
{{Main|Optical physics|Optical engineering}}
''Modern optics'' encompasses the areas of optical science and engineering that became popular in the 20th century. 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. Quantum optics is not just theoretical; some modern devices, such as lasers, have principles of operation that depend on quantum mechanics. Light detectors, such as [[photomultiplier]]s and [[channeltron]]s, respond to individual photons. Electronic [[image sensor]]s, such as [[Charge-coupled device|CCDs]], exhibit [[shot noise]] corresponding to the statistics of individual photon events. [[Light-emitting diode]]s and [[photovoltaic cell]]s, too, cannot be understood without quantum mechanics. In the study of these devices, quantum optics often overlaps with [[quantum electronics]].<ref>
Specialty areas of optics research include the study of how light interacts with specific materials as in [[crystal optics]] and [[metamaterial]]s. Other research focuses on the phenomenology of electromagnetic waves as in [[optical vortex|singular optics]], [[non-imaging optics]], [[non-linear optics]], statistical optics, and [[radiometry]]. Additionally, [[computer engineer]]s have taken an interest in [[integrated optics]], [[machine vision]], and [[photonic computing]] as possible components of the "next generation" of computers.<ref>{{cite book |last= McAulay |
Today, the pure science of optics is called optical science or [[optical physics]] to distinguish it from applied optical sciences, which are referred to as [[optical engineering]]. Prominent subfields of optical engineering include [[lighting|illumination engineering]], [[photonics]], and [[optoelectronics]] with practical applications like [[Optical lens design|lens design]], [[Fabrication and testing (optical components)|fabrication and testing of optical components]], and [[image processing]]. Some of these fields overlap, with nebulous boundaries between the subjects' terms that mean slightly different things in different parts of the world and in different areas of industry. A professional community of researchers in nonlinear optics has developed in the last several decades due to advances in laser technology.<ref>{{cite book |
</ref>
Line 284 ⟶ 270:
{{Main|Laser}}
[[File:Military laser experiment.jpg|thumb|Experiments such as this one with high-power [[laser]]s are part of the modern optics research.]]
A laser is a device that emits light, a kind of electromagnetic radiation, through a process called ''[[stimulated emission]]''. The term ''laser'' is an [[acronym]] for
[[File:The VLT’s Artificial Star.jpg|thumb|left|[[Very Large Telescope|VLT]]'s laser guide star<ref>{{cite news|title=The VLT's Artificial Star|url=http://www.eso.org/public/images/potw1425a/|access-date=25 June 2014|work=ESO Picture of the Week|url-status=live|archive-url=https://web.archive.org/web/20140703151816/http://www.eso.org/public/images/potw1425a/|archive-date=3 July 2014}}</ref>]]
Line 318 ⟶ 304:
{{for|the visual effects used in film, video, and computer graphics|visual effects}}
[[File:Ponzo illusion.gif|right|thumb|The Ponzo Illusion relies on the fact that parallel lines appear to converge as they approach infinity.]]
Optical illusions (also called visual illusions) are characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a [[percept]] that differs from the object being imaged. Optical illusions can be the result of a variety of phenomena including physical effects that create images that are different from the objects that make them, the physiological effects on the eyes and brain of excessive stimulation (e.g. brightness, tilt, colour, movement), and cognitive illusions where the eye and brain make [[unconscious inference]]s.<ref>{{cite web|url=http://www.livescience.com/strangenews/080602-foresee-future.html|title=Key to All Optical Illusions Discovered|author=J. Bryner|publisher=LiveScience
Cognitive illusions include some which result from the unconscious misapplication of certain optical principles. For example, the [[Ames room]], [[Hering illusion|Hering]], [[Müller-Lyer illusion|Müller-Lyer]], [[Orbison's illusion|Orbison]], [[Ponzo illusion|Ponzo]], [[Sander illusion|Sander]], and [[Wundt illusion]]s all rely on the suggestion of the appearance of distance by using converging and diverging lines, in the same way that parallel light rays (or indeed any set of parallel lines) appear to converge at a [[vanishing point]] at infinity in two-dimensionally rendered images with artistic perspective.<ref>[http://mathdl.maa.org/convergence/1/?pa=content&sa=viewDocument&nodeId=477&bodyId=598 Geometry of the Vanishing Point] {{webarchive|url=https://web.archive.org/web/20080622055904/http://mathdl.maa.org/convergence/1/?pa=content&sa=viewDocument&nodeId=477&bodyId=598 |date=2008-06-22 }} at [http://mathdl.maa.org/convergence/1/ Convergence] {{webarchive|url=https://web.archive.org/web/20070713083148/http://mathdl.maa.org/convergence/1/ |date=2007-07-13 }}</ref> This suggestion is also responsible for the famous [[moon illusion]] where the moon, despite having essentially the same angular size, appears much larger near the [[horizon]] than it does at [[zenith]].<ref>[http://facstaff.uww.edu/mccreadd/ "The Moon Illusion Explained"] {{webarchive|url=https://web.archive.org/web/20151204212728/http://facstaff.uww.edu/mccreadd/ |date=2015-12-04 }}, Don McCready, University of Wisconsin-Whitewater</ref> This illusion so confounded [[Ptolemy of Alexandria|Ptolemy]] that he incorrectly attributed it to atmospheric refraction when he described it in his treatise, ''[[Optics (Ptolemy)|Optics]]''.<ref name=Ptolemy />
Line 329 ⟶ 315:
{{Main|Optical instruments}}
Single lenses have a variety of applications including [[photographic lens]]es, corrective lenses, and magnifying glasses while single mirrors are used in parabolic reflectors and [[rear-view mirror]]s. Combining a number of mirrors, prisms, and lenses produces compound optical instruments which have practical uses. For example, a [[periscope]] is simply two plane mirrors aligned to allow for viewing around obstructions. The most famous compound optical instruments in science are the microscope and the telescope which were both invented by the Dutch in the late 16th century.
Microscopes were first developed with just two lenses: an [[objective lens]] and an [[eyepiece]]. The objective lens is essentially a magnifying glass and was designed with a very small focal length while the eyepiece generally has a longer focal length. This has the effect of producing magnified images of close objects. Generally, an additional source of illumination is used since magnified images are dimmer due to the [[conservation of energy]] and the spreading of light rays over a larger surface area. Modern microscopes, known as ''compound microscopes'' have many lenses in them (typically four) to optimize the functionality and enhance image stability.
The first telescopes, called refracting telescopes, were also developed with a single objective and eyepiece lens. In contrast to the microscope, the objective lens of the telescope was designed with a large focal length to avoid optical aberrations. The objective focuses an image of a distant object at its focal point which is adjusted to be at the focal point of an eyepiece of a much smaller focal length. The main goal of a telescope is not necessarily magnification, but rather the collection of light which is determined by the physical size of the objective lens. Thus, telescopes are normally indicated by the diameters of their objectives rather than by the magnification which can be changed by switching eyepieces. Because the magnification of a telescope is equal to the focal length of the objective divided by the focal length of the eyepiece, smaller focal-length eyepieces cause greater magnification.
Since crafting large lenses is much more difficult than crafting large mirrors, most modern telescopes are ''[[reflecting telescope]]s'', that is, telescopes that use a primary mirror rather than an objective lens. The same general optical considerations apply to reflecting telescopes that applied to refracting telescopes, namely, the larger the primary mirror, the more light collected, and the magnification is still equal to the focal length of the primary mirror divided by the focal length of the eyepiece. Professional telescopes generally do not have eyepieces and instead place an instrument (often a charge-coupled device) at the focal point instead.
===Photography===
Line 379 ⟶ 365:
Mirages are optical phenomena in which light rays are bent due to thermal variations in the refraction index of air, producing displaced or heavily distorted images of distant objects. Other dramatic optical phenomena associated with this include the [[Novaya Zemlya effect]] where the sun appears to rise earlier than predicted with a distorted shape. A spectacular form of refraction occurs with a [[inversion (meteorology)|temperature inversion]] called the [[Fata Morgana (mirage)|Fata Morgana]] where objects on the horizon or even beyond the horizon, such as islands, cliffs, ships or icebergs, appear elongated and elevated, like "fairy tale castles".<ref>{{cite web|url=http://mintaka.sdsu.edu/GF/mirages/mirintro.html|title=An Introduction to Mirages|author=A. Young|url-status=live|archive-url=https://web.archive.org/web/20100110045709/http://mintaka.sdsu.edu/GF/mirages/mirintro.html|archive-date=2010-01-10}}</ref>
Rainbows are the result of a combination of internal reflection and dispersive refraction of light in raindrops. A single reflection off the backs of an array of raindrops produces a rainbow with an angular size on the sky that ranges from 40° to 42° with red on the outside. Double rainbows are produced by two internal reflections with angular size of 50.5° to 54° with violet on the outside. Because rainbows are seen with the sun 180° away from the centre of the rainbow, rainbows are more prominent the closer the sun is to the horizon.
==See also==
Line 391 ⟶ 377:
{{Reflist}}
===Works cited===
;Further reading▼
{{refbegin |30em |indent= yes}}
* {{cite book |isbn=978-1-139-64340-5 |title=[[Principles of Optics]] |last1=Born |first1=Max |last2=Wolf |first2=Emil |year=2002 |publisher=Cambridge University Press }}▼
* {{cite book |
* {{cite book |last1= Young |first1= Hugh D. |last2= Freedman |first2= Roger A. |date= 2020 |title= University Physics: Extended Version With Modern Physics |edition= 15th |publisher= Pearson Education |isbn= 978-1-292-31473-0 }}
* {{cite book |isbn=978-0-534-40842-8 |title=Physics for scientists and engineers |last1=Serway |first1=Raymond A. |year=2004 |publisher=Thomson-Brooks/Cole |location=Belmont, CA |last2=Jewett |first2=John W. |edition=6, illustrated |url=https://archive.org/details/physicssciengv2p00serw }}▼
{{refend}}
* {{cite book |isbn=978-0-7167-0810-0 |title=Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics |last1=Tipler |first1=Paul A. |year=2004 |publisher=W.H. Freeman |last2=Mosca |first2=Gene |volume=2 }}▼
▲* {{cite book
* {{cite book |last1= Fowles |first1= Grant R. |date= 1975 |title= Introduction to Modern Optics |edition= 4th |publisher= Addison-Wesley Longman }}
* {{cite book |last1= Lipson |first1= Stephen G. |last2= Lipson |first2= Henry |last3= Tannhauser |first3= David Stefan |date= 1995 |title= Optical Physics |publisher=Cambridge University Press |isbn= 978-0-521-43631-1 }}
▲* {{cite book |last1= Serway |first1= Raymond A. |last2= Jewett |first2= John W. |date= 2004 |isbn= 978-0-534-40842-8 |title= Physics for
▲* {{cite book |
==External links==
| |||