Editing Optics (section)

===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}}