Editing Optics (section)

==Applications==
Optics is part of everyday life. The ubiquity of [[visual system]]s in biology indicates the central role optics plays as the science of one of the [[sense|five senses]]. Many people benefit from [[eyeglasses]] or [[contact lenses]], and optics are integral to the functioning of many consumer goods including [[cameras]]. Rainbows and mirages are examples of optical phenomena. [[Optical communication]] provides the backbone for both the [[Internet]] and modern [[telephony]].

===Human eye===
[[File:Eye-diagram no circles border.svg|upright=1.35|thumb|right|Model of a human eye. Features mentioned in this article are 1. [[vitreous humour]] 3. [[ciliary muscle]], 6. [[pupil]], 7. [[anterior chamber]], 8. [[cornea]], 10. [[lens cortex]], 22. [[optic nerve]], 26. [[Fovea centralis|fovea]], 30. [[retina]].]]
{{Main|Human eye|Photometry (optics)}}
[[File:Human eye with limbal ring, anterior view.jpg|thumb|The [[human eye]] is a living optical device. The [[Iris (anatomy)|iris]] (light brown region), [[pupil]] (black circle in the centre), and [[sclera]] (white surrounding area) are visible in this image, along with the [[eyelid]]s and [[eyelash]]es which protect the eye]]
The human eye functions by focusing light onto a layer of [[photoreceptor cell]]s called the retina, which forms the inner lining of the back of the eye. The focusing is accomplished by a series of transparent media. Light entering the eye passes first through the cornea, which provides much of the eye's optical power. The light then continues through the fluid just behind the cornea—the [[anterior chamber]], then passes through the [[pupil]]. The light then passes through the [[lens (anatomy)|lens]], which focuses the light further and allows adjustment of focus. The light then passes through the main body of fluid in the eye—the [[vitreous humour]], and reaches the retina. The cells in the retina line the back of the eye, except for where the optic nerve exits; this results in a [[Blind spot (vision)|blind spot]].

There are two types of photoreceptor cells, rods and cones, which are sensitive to different aspects of light.<ref name=eyeoptics>{{cite book|author1=D. Atchison  |author2=G. Smith |name-list-style=amp |title=Optics of the Human Eye|year=2000|isbn=978-0-7506-3775-6|publisher=Elsevier}}</ref> Rod cells are sensitive to the intensity of light over a wide frequency range, thus are responsible for [[scotopic vision|black-and-white vision]]. Rod cells are not present on the fovea, the area of the retina responsible for central vision, and are not as responsive as cone cells to spatial and temporal changes in light. There are, however, twenty times more rod cells than cone cells in the retina because the rod cells are present across a wider area. Because of their wider distribution, rods are responsible for [[peripheral vision]].<ref name="Kandel">{{cite book|author1=E.R. Kandel|author2=J.H. Schwartz|author3=T.M. Jessell|title=Principles of Neural Science|edition=4th|year=2000|publisher=McGraw-Hill|place=New York|isbn=978-0-8385-7701-1|pages=[https://archive.org/details/isbn_9780838577011/page/507 507–513]|url=https://archive.org/details/isbn_9780838577011/page/507}}</ref>

In contrast, cone cells are less sensitive to the overall intensity of light, but come in three varieties that are sensitive to different frequency-ranges and thus are used in the perception of [[colour]] and [[photopic vision]]. Cone cells are highly concentrated in the fovea and have a high visual acuity meaning that they are better at spatial resolution than rod cells. Since cone cells are not as sensitive to dim light as rod cells, most [[night vision]] is limited to rod cells. Likewise, since cone cells are in the fovea, central vision (including the vision needed to do most reading, fine detail work such as sewing, or careful examination of objects) is done by cone cells.<ref name="Kandel" />

Ciliary muscles around the lens allow the eye's focus to be adjusted. This process is known as [[Accommodation (eye)|accommodation]]. The [[near point]] and [[far point]] define the nearest and farthest distances from the eye at which an object can be brought into sharp focus. For a person with normal vision, the far point is located at infinity. The near point's location depends on how much the muscles can increase the curvature of the lens, and how inflexible the lens has become with age. [[Optometrist]]s, [[ophthalmologist]]s, and [[optician]]s usually consider an appropriate near point to be closer than normal reading distance—approximately 25&nbsp;cm.<ref name=eyeoptics />

Defects in vision can be explained using optical principles. As people age, the lens becomes less flexible and the near point recedes from the eye, a condition known as [[presbyopia]]. Similarly, people suffering from [[hyperopia]] cannot decrease the focal length of their lens enough to allow for nearby objects to be imaged on their retina. Conversely, people who cannot increase the focal length of their lens enough to allow for distant objects to be imaged on the retina suffer from [[myopia]] and have a far point that is considerably closer than infinity. A condition known as [[Astigmatism (eye)|astigmatism]] results when the cornea is not spherical but instead is more curved in one direction. This causes horizontally extended objects to be focused on different parts of the retina than vertically extended objects, and results in distorted images.<ref name=eyeoptics />

All of these conditions can be corrected using [[corrective lens]]es. For presbyopia and hyperopia, a [[converging lens]] provides the extra curvature necessary to bring the near point closer to the eye while for myopia a [[diverging lens]] provides the curvature necessary to send the far point to infinity. Astigmatism is corrected with a [[cylinder (geometry)|cylindrical surface]] lens that curves more strongly in one direction than in another, compensating for the non-uniformity of the cornea.<ref name=lensdesign>{{cite web|url=http://www.opticampus.com/cecourse.php?url=lens_design/&OPTICAMP=f1e4252df70c63961503c46d0c8d8b60|title=Ophthalmic Lens Design|author=D. Meister|publisher=OptiCampus.com|access-date=November 12, 2008|url-status=live|archive-url=https://web.archive.org/web/20081227021113/http://www.opticampus.com/cecourse.php?url=lens_design%2F&OPTICAMP=f1e4252df70c63961503c46d0c8d8b60|archive-date=December 27, 2008}}</ref>

The optical power of corrective lenses is measured in [[diopter]]s, a value equal to the [[multiplicative inverse|reciprocal]] of the focal length measured in metres; with a positive focal length corresponding to a converging lens and a negative focal length corresponding to a diverging lens. For lenses that correct for astigmatism as well, three numbers are given: one for the spherical power, one for the cylindrical power, and one for the angle of orientation of the astigmatism.<ref name=lensdesign />

====Visual effects<!--linked from 'Practical effect'-->====
{{Main|Optical illusion|Perspective (graphical)}}
{{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 |date=2008-06-02|url-status=live|archive-url=https://web.archive.org/web/20080905122802/http://www.livescience.com/strangenews/080602-foresee-future.html|archive-date=2008-09-05}}</ref>

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 />

Another type of optical illusion exploits broken patterns to trick the mind into perceiving symmetries or asymmetries that are not present. Examples include the [[Café wall illusion|café wall]], [[Ehrenstein illusion|Ehrenstein]], [[Fraser spiral illusion|Fraser spiral]], [[Poggendorff illusion|Poggendorff]], and [[Zöllner illusion]]s. Related, but not strictly illusions, are patterns that occur due to the superimposition of periodic structures. For example, [[Transparency (optics)|transparent]] tissues with a grid structure produce shapes known as [[moiré pattern]]s, while the superimposition of periodic transparent patterns comprising parallel opaque lines or curves produces [[line moiré]] patterns.<ref>{{cite book|title=Energy Minimization Methods in Computer Vision and Pattern Recognition|author1=A.K. Jain |author2=M. Figueiredo |author3=J. Zerubia |publisher=Springer
|year=2001|url = https://books.google.com/books?id=yb8otde21fcC&pg=RA1-PA198|isbn=978-3-540-42523-6}}</ref>

====Optical instruments====
[[File:Table of Opticks, Cyclopaedia, Volume 2.jpg|thumb|right|upright=1.35|Illustrations of various optical instruments from the 1728 ''[[Cyclopaedia, or an Universal Dictionary of Arts and Sciences|Cyclopaedia]]'']]
{{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.{{sfnp|Young|Freedman|2020|pp=1171–1175}}

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.{{sfnp|Young|Freedman|2020|pp=1171–1173}} A slightly different variety of microscope, the [[comparison microscope]], looks at side-by-side images to produce a [[Stereoscopy|stereoscopic]] [[binocular vision|binocular]] view that appears three dimensional when used by humans.<ref>{{cite web|url=http://www.microscopyu.com/articles/stereomicroscopy/stereointro.html|title=Introduction to Stereomicroscopy|author1=P.E. Nothnagle|author2=W. Chambers|author3=M.W. Davidson|publisher=Nikon MicroscopyU|url-status=live|archive-url=https://web.archive.org/web/20110916115256/http://www.microscopyu.com/articles/stereomicroscopy/stereointro.html|archive-date=2011-09-16}}</ref>

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.{{sfnp|Young|Freedman|2020|p=1174}}

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.{{sfnp|Young|Freedman|2020|pp=1175}}

===Photography===
{{Main|Science of photography}}
[[File:Jonquil flowers at f32.jpg|thumb|right|upright=1.35|Photograph taken with aperture {{f/}}32]]
[[File:Jonquil flowers at f5.jpg|thumb|right|upright=1.35|Photograph taken with aperture {{f/}}5]]
The optics of photography involves both lenses and the medium in which the electromagnetic radiation is recorded, whether it be a [[photographic plates|plate]], [[photographic film|film]], or charge-coupled device. Photographers must consider the [[Reciprocity (photography)|reciprocity]] of the camera and the shot which is summarized by the relation

:Exposure ∝ ApertureArea × ExposureTime × SceneLuminance<ref>
{{cite book
|title=Investigations on the Theory of the Photographic Process
|author1=Samuel Edward Sheppard  |author2=Charles Edward Kenneth Mees
 |name-list-style=amp |publisher=Longmans, Green and Co
|year=1907
|page = [https://archive.org/details/investigationso01meesgoog/page/n232 214]
|url = https://archive.org/details/investigationso01meesgoog
}}</ref>

In other words, the smaller the aperture (giving greater depth of focus), the less light coming in, so the length of time has to be increased (leading to possible blurriness if motion occurs). An example of the use of the law of reciprocity is the [[Sunny 16 rule]] which gives a rough estimate for the settings needed to estimate the proper [[exposure (photography)|exposure]] in daylight.<ref>{{cite book|title=Mastering Black-and-White Photography|author=B.J. Suess|publisher=Allworth Communications|year=2003|isbn=978-1-58115-306-4|url=https://books.google.com/books?id=7LaRPNINH_YC&pg=PT112}}</ref>

A camera's aperture is measured by a unitless number called the [[f-number]] or f-stop, {{f/}}#, often notated as <math>N</math>, and given by
:<math>f/\# = N = \frac fD \ </math>
where <math>f</math> is the focal length, and <math>D</math> is the diameter of the entrance pupil. By convention, "{{f/}}#" is treated as a single symbol, and specific values of {{f/}}# are written by replacing the [[number sign]] with the value. The two ways to increase the f-stop are to either decrease the diameter of the entrance pupil or change to a longer focal length (in the case of a [[zoom lens]], this can be done by simply adjusting the lens). Higher f-numbers also have a larger [[depth of field]] due to the lens approaching the limit of a pinhole camera which is able to focus all images perfectly, regardless of distance, but requires very long exposure times.<ref>{{cite book|title=Basic Photography|author=M.J. Langford|isbn=978-0-240-51592-2|year=2000|publisher=Focal Press|url=https://archive.org/details/basicphotography00lang}}</ref>

The field of view that the lens will provide changes with the focal length of the lens. There are three basic classifications based on the relationship to the diagonal size of the film or sensor size of the camera to the focal length of the lens:<ref name="Bruce Warren, Photography, page 71">{{cite book |first=Bruce |last=Warren |title=Photography |url=https://books.google.com/books?id=sbdGeFem1zwC&pg=PA71 |year=2001 |publisher=Cengage Learning |isbn=978-0-7668-1777-7 |page=71 |url-status=live |archive-url=https://web.archive.org/web/20160819020407/https://books.google.com/books?id=sbdGeFem1zwC&pg=PA71 |archive-date=2016-08-19 }}</ref>
* [[Normal lens]]: angle of view of about 50° (called ''normal'' because this angle considered roughly equivalent to human vision<ref name="Bruce Warren, Photography, page 71"/>) and a focal length approximately equal to the diagonal of the film or sensor.<ref>{{cite book|title=View Camera Technique|author=Leslie D. Stroebel|publisher=Focal Press|year=1999|isbn=978-0-240-80345-6|url=https://books.google.com/books?id=71zxDuunAvMC&pg=PA136}}</ref> <!-- It's generally accepted that 50mm is a bit longer than "normal" for 35mm film; I've based this on 75mm with 6×4cm. Maybe the angle of width, rather than diagonal, is better? -->
* [[Wide-angle lens]]: angle of view wider than 60° and focal length shorter than a normal lens.<ref>{{cite book|title=Using the View Camera|author=S. Simmons|publisher=Amphoto Books|year=1992|isbn=978-0-8174-6353-3|page=35}}</ref>
* [[Long focus lens]]: angle of view narrower than a normal lens. This is any lens with a focal length longer than the diagonal measure of the film or sensor.<ref>{{cite book |author=Sidney F. Ray |title=Applied Photographic Optics: Lenses and Optical Systems for Photography, Film, Video, Electronic and Digital Imaging |url=https://books.google.com/books?id=cuzYl4hx-B8C&pg=PA294 |year=2002 |publisher=Focal Press |isbn=978-0-240-51540-3 |page=294 |url-status=live |archive-url=https://web.archive.org/web/20160819003154/https://books.google.com/books?id=cuzYl4hx-B8C&pg=PA294 |archive-date=2016-08-19 }}</ref> The most common type of long focus lens is the [[telephoto lens]], a design that uses a special ''telephoto group'' to be physically shorter than its focal length.<ref>{{cite book|url = https://books.google.com/books?id=zqkdNwRxSooC&pg=PA109 |title=The New York Times Guide to Essential Knowledge|author=New York Times Staff|isbn=978-0-312-31367-8|year=2004|publisher=Macmillan}}</ref>

Modern zoom lenses may have some or all of these attributes.

The absolute value for the exposure time required depends on how [[sensitometry|sensitive]] to light the medium being used is (measured by the [[film speed]], or, for digital media, by the [[quantum efficiency]]).<ref>{{cite book|title=Principles of Radiographic Imaging: An Art and a Science|author1=R.R. Carlton |author2=A. McKenna Adler |publisher=Thomson Delmar Learning|year=2000|isbn=978-0-7668-1300-7|url = https://books.google.com/books?id=oA-eBHsapX8C&pg=PA318}}</ref> Early photography used media that had very low light sensitivity, and so exposure times had to be long even for very bright shots. As technology has improved, so has the sensitivity through film cameras and digital cameras.<ref>{{cite book|author=W. Crawford|title=The Keepers of Light: A History and Working Guide to Early Photographic Processes|year=1979|publisher=Morgan & Morgan|location=Dobbs Ferry, NY|isbn=978-0-87100-158-0|page=20}}</ref>

Other results from physical and geometrical optics apply to camera optics. For example, the maximum resolution capability of a particular camera set-up is determined by the [[diffraction limit]] associated with the pupil size and given, roughly, by the Rayleigh criterion.<ref>{{cite book|author=J.M. Cowley|year=1975|title=Diffraction physics|location=Amsterdam|publisher=North-Holland|isbn=978-0-444-10791-6}}</ref>

===Atmospheric optics===
{{main|Atmospheric optics}}

[[File:Firesunset2edit.jpg|thumb|right|upright=1.35|A colourful sky is often due to scattering of light off particulates and pollution, as in this photograph of a sunset during the [[October 2007 California wildfires]].]]

The unique optical properties of the atmosphere cause a wide range of spectacular optical phenomena. The blue colour of the sky is a direct result of Rayleigh scattering which redirects higher frequency (blue) sunlight back into the field of view of the observer. Because blue light is scattered more easily than red light, the sun takes on a reddish hue when it is observed through a thick atmosphere, as during a [[sunrise]] or [[sunset]]. Additional particulate matter in the sky can scatter different colours at different angles creating colourful glowing skies at dusk and dawn. Scattering off of ice crystals and other particles in the atmosphere are responsible for [[halo (optical phenomenon)|halos]], [[afterglow]]s, [[Corona (meteorology)|coronas]], [[Crepuscular rays|rays of sunlight]], and [[sun dog]]s. The variation in these kinds of phenomena is due to different particle sizes and geometries.<ref name="autogenerated1">{{cite book|author=C.D. Ahrens|year=1994|title=Meteorology Today: an introduction to weather, climate, and the environment|edition=5th|pages=[https://archive.org/details/meteorologytoday00ahre/page/88 88–89]|publisher=West Publishing Company|isbn=978-0-314-02779-5|url=https://archive.org/details/meteorologytoday00ahre/page/88}}</ref>

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.{{sfnp|Young|Freedman|2020|pp=1117–1118}}