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THE EYE: OPTICS OF VISIONREFRACTION OF LIGHTRefractive Index of a Transparent SubstanceLIGHT RAYS TRAVEL: 300,000 km/sec – air 200,000km/sec – particular type of glassREFRACTIVE INDEX OF AIR – 1.00Refraction of Light Rays at an Interface Between Two Media with Different Refractive Indices“THE DIRECTION IN WHICH LIGHT TRAVELS IS ALWAYS PERPENDICULAR TO THE PLANE OF THE WAVE FRONT , THE DIRECTION OF TRAVEL OF LIGHT BEAM BENDS DOWNWARD”REFRACTON- the bending of light rays at an angulated interface

APPLICATION OF REFRACTIVE PRINCIPLES TO LENSESCONVEX LENS FOCUSES LIGHT RAYSConvergence – the outer rays bend more and more toward the centre.If the lens has exactly the proper curvature parallel light rays passing through each part of the lens will be bent exactly enough so that all the rays will pass through a single point which is called FOCAL POINT.

CYLINDRICAL LENS BENDS LIGHT RAYS IN ONLY ONE PLANE – COMPARISON WITH SPHERICAL LENSESCONCAVE LENS DIVERGES LIGHT RAYS

COMBINATION OF TWO CYLINDRICAL LENSES AT RIGHT ANGLES EQUALS A SPHERICAL LENS

FORMATION OF AN IMAGE BY A CONVEX LENS

MEASUREMENT OF THE REFRACTIVE POWER OF THE LENS- “DIOPTER”The more a lens bends light rays, the greater is its “refractive power”. This refractive power is measured in terms of DIOPTERS.

OPTICS OF THE EYETHE EYE AS A CAMERAThe eye is optically equivalent to the usual photographic camera. It has a lens system, a variable aperture system (pupil), and a retina that corresponds to the film. LENS SYSTEM: Composed of four refractive interfaces:The interface between air and the anterior surface of the corneaThe interface between the posterior surface of the cornea and aqueous humor

3. The interface between the aqueous humor and the anterior surface of the lens 4. The interface between the posterior surface of the lens and the vitreous humorThe internal index of air is 1; the cornea, 1.38; the aqueuoshumor, 1.33; the crystalline lens, 1.40 and the vitreous humor, 1.34.REDUCED EYE If all the refractive surfaces of the eye are algebraically added together and then considered to be one single lens, the optics of the normal eye may be simplfied and represented schematically as a “reduced eye”. This is useful in simple calculatons.

FORMATION OF AN IMAGE ON THE RETINAIn the same manner that a glass lens can focus an image on a sheet of paper, the lens system of the eye can focus an image on the retina. The image is inverted and reversed with respect to the object. However, the mind perceives objects in the upright position despite the upside-down orientation on the retina because the brain is trained to consider an inverted image as normal.

MECHANISM OF ACCOMODATIONIn the young person, the lens is composed of a strong elastic capsule filled with viscous, proteinaceous , but transparent fluid.

When the lens is in relaxed state with no tension on its capsule, it assumes an almost spherical shape, owing mainly to the elastic retraction of the lens capsule.

Located at the attachments of the lens ligaments to the eyeball is the ciliary muscle, which itself has two separate sets of smooth muscle fibers, meridonalfibers and circular fibers.

MERIDONAL FIBERS – extend anteriorly from the peripheral ends of the suspensory ligaments to the corneoscleral junction. When the muscle fibers contract, the peripheral insertions of the lens ligaments are pulled forward and medially toward the cornea, thereby releasing tension on the lens.CIRCULAR FIBERS – arranged circularly all the way around the ligament attachments so that when they contract, the sphincter-like action occurs, decreasing the diameter of the circle of the ligament attachments; this also allows the ligaments to pull less on the lens capsule

ACCOMODATION IS CONTROLLED BY THE PARASYMPATHETIC NERVESPRESBYOPIA - as a person grows older, the lens grows larger and thicker and becomes far less elastic, partly because of progressive denaturation of the lens proteins

PUPILLARY DIAMETER The amount of light that enters the eye through the pupil is proportional to the area of the pupil or to the square of the diameter of the pupil. The pupil of the human eye can become as small as about 1.5 millimeters and as large as 8 millimeters in diameter. THE DEPTH OF FOCUS OF THE LENS SYSTEM INCREASES WITH DECREASING PUPILLARY DIAMETERERRORS OF REFRACTIONEMMETROPIA ( Normal Vision) The eye is considered to be normal, or “emmetropic” if paralllel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed.HYPEROPIA (Farsightedness)This is due to an eyeball that is too short or occasionally to a lens system that is too weak.MYOPIA ( Nearsightedness)This is due to an eyeball that is too long, but it can result from too much refractive power in the lens system of the eye.

CORRECTION OF MYOPIA AND HYPEROPIA BY USE OF LENSESIn myopia, the excessive refractive power can be neutralized by placing in front of the eye a concave spherical lens, which will diverge lens.

In hyperopia, the abnormal vision can be corrected by adding refractive power using a convex lens in front of the eye.ASTIGMATISM*ASTIGMATISM - is a refractive error of the eye that causes the visual image in one plane to focus at a different distance from that of the plane at right angles. This often results from too great curvature of the cornea ion one of its planes. The accommodative power of the eye can never compensate for astigmatism because during accommodation, the curvature of the eye lens changes approximately equally in both planes.CORRECTION OF ASTIGMATISM – with a cylindrical lens

CORRECTION OF OPTICAL ABNORMALITIES BY THE USE OF CONTACT LENSESA special feature of the contact lens is that it nullifies almost entirely the refraction that normally occurs at the anterior surface of the cornea. The reason for this is that the tears between the contact lens and the cornea have a refractive index almost equal to that of the cornea so that no longer does the anterior surface of the cornea play a significant role in the eye’s optical system.

The refraction of this contact lens substitute for the cornea’s usual refraction. CATARACTSA cataract is a cloudy or opaque area(s) in the lens. In the early stage of the cataract formation, the protein in some of the lens fibers become denatured. Later, these same proteins coagulate to form opaque areas in place of the normal transparent protein fiber.

VISUAL ACUITYThe normal visual acuity of the human eye for discriminating between point sources of the light is about 25 seconds of arc. NORMAL VISION- 20/20DETERMINATION OF DISTANCE OF AN OBJECT FROM THE EYE – DEPTH PERCEPTIONDetermination of Distance by Sizes of Retinal Images of Known ObjectsDetermination of Distance by Moving ParallaxDetermination of Distance by Stereopsis- Binocular Vision

OPHTHALMOSCOPEThe ophthalmoscope is an instrument through which an observer can look into another person’s eye and see the retina with clarity.

FLUID SYSTEM OF THE EYE – INTRAOCULAR FLUIDIntraocular fluid – fluid in the eye that maintains the sufficient pressure in the eyeball to keep it distended.

Aqueous humor – lies in front and to the sides of the lens;free flowing fluid and continually being formed and reabsorbedVitreous humor – lies between the posterior surface of the lens and the retina; gelatinous mass held together by a fine fibrillar networkFORMATION OF AQUEOUS HUMOR BY THE CILIARY BODYAqueous humor is formed in the eye at an average rate of 2 to 3 microliters each minute. Essentially all of it is secreted by the ciliary processes, which are linear folds that project from the ciliary body into the space behind the iris where the lens ligaments and ciliary muscle attach to the eyeball.Aqueous humor is formed almost entirely as an active secretion of the epithelium lining the ciliary processes. It begins with active transport of sodium ions into the spaces between epithelial cells.

INTRAOCULAR PRESSUREAVERAGE NORMAL IOP – 15mm Hg with a range from 12 to 20TONOMETRY: The measurement of tension or pressure inside the eye (IOP)

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The eye. physio.lourdes

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