e403 - Applications

5/21/2018 e403 - Applications

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The Human Eye

In order to see, we must have light. While we don't fully understand all the different

properties of light, we do have an idea of how light travels.

A light ray can be deflected, reflected, bent or absorbed, depending on the different

substances it encounters.

When light travels through water or a lens, for example, its path is bent or refracted.

Certain eye structures have refractive properties similar to water or lenses and canbend light rays into a precise point of focus essential for sharp vision.

Most refraction in the eye occurs when light rays travel through the curved, clearfront surface of the eye (cornea). The eye's natural (crystalline)lens also bends light

rays. Even the eyestear film and internal fluids (aqueous humor andvitreous) have

refractive abilities.

The process of vision

begins when light raysthat reflect off objects

and travel through theeye's optical system

are refracted andfocused into a point of

sharp focus.

For good vision, this

focus point must be onthe retina. The retina is

the tissue that lines theinside of the back of the eye, where light-sensitive cells (photoreceptors) capture

images in much the same way that film in a camera does when exposed to light.These images then are transmitted through the eye's optic nerve to the brain for

interpretation.

Just as a camera's aperture (called the diaphragm) is used to adjust the amount oflight needed to expose film in just the right way, the eye'spupil widens or constricts

to control the amount of light that reaches the retina.In dark conditions, the pupil widens. In bright conditions, the pupil constricts.

The Astronomical Telescope

The astronomical telescope makes use of two positive lenses: the objective, whichforms the image of a distant object at itsfocal length,and the eyepiece, which acts
http://www.all-about-vision.com/glossary/definition.php?defID=439http://www.all-about-vision.com/glossary/definition.php?defID=446http://www.all-about-vision.com/glossary/definition.php?defID=453http://www.all-about-vision.com/glossary/definition.php?defID=259http://www.all-about-vision.com/glossary/definition.php?defID=460http://www.all-about-vision.com/glossary/definition.php?defID=475http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/foclen.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/foclen.html#c1http://www.all-about-vision.com/glossary/definition.php?defID=475http://www.all-about-vision.com/glossary/definition.php?defID=460http://www.all-about-vision.com/glossary/definition.php?defID=259http://www.all-about-vision.com/glossary/definition.php?defID=453http://www.all-about-vision.com/glossary/definition.php?defID=446http://www.all-about-vision.com/glossary/definition.php?defID=439


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as a simple magnifier with which to view the image formed by the objective. Itslength is equal to the sum of the focal lengths of the objective and eyepiece, and itsangular magnification is -fo/fe, giving an inverted image.

The astronomical telescope can be used for terrestrial viewing, but seeing the imageupside down is a definite inconvenience. Viewing stars upside down is no problem.Another inconvenience for terrestrial viewing is the length of the astronomicaltelescope, equal to the sum of the focal lengths of the objective and eyepiece lenses.A shorter telescope with upright viewing is theGalilean telescope.

Galilean Telescope

The Galilean or terrestrial telescope uses a positive objective and a negativeeyepiece. It gives erect images and is shorter than theastronomical telescope with

the same power. Itsangular magnification is -fo/fe.

The image below shows parallel rays from twohelium-neon lasers passing through aGalilean telescope made from an objective with f=30cm and an eyepiece with f=-10cm.
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/simmag.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lasgas.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lasgas.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/simmag.html#c1


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With the lenses placed 20 cm = fo+fe apart, the parallel input rays are renderedparallel again by the eyepiece lens, giving an image at infinity. This shows one of theuses of Galilean telescopes. It is useful as a collimator that takes a large beam ofparallel light and reduces the size of the beam, keeping the rays parallel. The angularmagnification of this Galilean telescope is 3. The beams of the helium-neon laserswere made visible with a spray can of artificial smoke.

Microscope Optics

The basic light path of the microscope can be clearly seen at the left. Light from thebulb in the base is focused by the

collector lenses in the base and sentupward, via a mirror or prism, as anilluminating cone of light which fills

the sub-stage condenser with light.

The condenser then focuses thelight and the image of the fields

diaphragm on the specimen. If theaperture diaphragm is set properly

the emerging light will fill theobjective and give maximum

resolution.

The preliminary image produced by

the objective is deflected by theprism into the eye tubes and then it

is further magnified by theeyepieces which project the image

into the eye, or if fitted into thecamera. The adjustment of the

condenser and the field andaperture diaphragms can be found

under the discussion on Khlerillumination.
http://web.uvic.ca/ail/techniques/zeiss_universal.htmlhttp://web.uvic.ca/ail/techniques/zeiss_universal.htmlhttp://web.uvic.ca/ail/techniques/zeiss_universal.htmlhttp://web.uvic.ca/ail/techniques/zeiss_universal.html


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The illustration above shows aschematic of the optics and light

paths of the microscope. Thecondenser focuses the image of

the field diaphragm into the

specimen plane and the plane ofthe eyepiece field stop. It alsofocuses the filament of the lamp

into the plane of the aperturediaphragm and the objective exit

pupil. The objective produces aprimary or intermediate image of

the specimen which is furthermagnified by the eyepiece and

projected into the eye or camera.

The condenser plays a critical rolein image formation. Highlycorrected condensers are complex

and are made of a number oflenses as seen below. Like a

microscope objective, acondenser has a numerical

aperture and it should equal orbetter that of the highest

magnification objective beingused. The wavelength of light

used (which can be selected by afilter), the objective numerical

aperture and the condensernumerical aperture all affect the

resolution of the instrumentaccording to the formulae below.

SOURCES:

http://www.allaboutvision.com/eye-exam/refraction.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html

http://web.uvic.ca/ail/techniques/scope_basics.html
http://www.allaboutvision.com/eye-exam/refraction.htmhttp://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.htmlhttp://web.uvic.ca/ail/techniques/scope_basics.htmlhttp://web.uvic.ca/ail/techniques/scope_basics.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.htmlhttp://www.allaboutvision.com/eye-exam/refraction.htm

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e403 - Applications