Eye- Structure and Refraction

Prof. K. Sivapalan

Jan. 2006 Eye- structure and optics 2

Structure of the Eye.• Sclera.• Cornea.• Conjunctiva.• Choroid.• Retina• Iris and pupil.• Ciliary body, muscles and

lens.• Anterior and posterior

chambers.• Vitreous• Optic disc, macula.

Jan. 2006 Eye- structure and optics 3

Lacrimal gland and tearing.• Lacrimal gland secretes

tear which flows through lacrimal duct of the upper eye lid.

• It moistens the cornea and washes it- removing dust.

• Lower eye lids collect the tears which go through naso-lacrimal duct.

• Increased by parasympathetic.

• Xerophthalmia- damage to cornea.

Jan. 2006 Eye- structure and optics 4

Nutrition of Eye.• Retina gets nutrients from

retinal artery that enters eye with optic nerve.

• Fovea has no vessols. Nurished by diffusion from choroid.

• Other structures get the nutrients from vessels in the choroid.

• There are no vessels in the anterior chamber, lens, posterior chamber and vitreous.

Jan. 2006 Eye- structure and optics 5

Vitreous and Aqueous Humor.

• Vitreous is clear gelatinous material through which nutritious material diffuse.

• The space between the lens and the cornea is divided into anterior and posterior chambers by the iris and filled with aqueous humor.

Jan. 2006 Eye- structure and optics 6

Aqueous humor.

• Ciliary body has ciliary processes in the posterior chamber where aqueous humor is secreted by active transport of sodium and other nutrients.

• The fluid passes through the pupil into anterior chamber and into the angle between the cornea and the iris.

• It is absorbed through the trabeculae into the Chanal of Schlemm and into veins.

• It is formed at a rate of 2.5 μL / min. and is responsible for the intra-ocular pressure of 15 [10-20] mm Hg and nourishes the cornea and lens.

Jan. 2006 Eye- structure and optics 7

Abnormalities of Intra-ocular Pressure.

• Dehydration reduces intra-ocular pressure.• It can result in abnormal curvatures in cornea.• Obstruction to absorption results in increase in

pressure [Glaucoma]• Increased pressure compresses retina, optic

nerve and retinal vessels.• The optic nerve is affected at the optic disc

because it is the weak point in the sclera. • It results in loss of vision in peripheral field-

conical vision at early stages. This can progress to total blindness.

Jan. 2006 Eye- structure and optics 8

Principles of Optics.• Light rays are refracted when they pass from one

medium to another.• Parallel rays [more than 6 meters] striking on biconvex

lens are refracted to principal focus.• The principal focus is in the principal axis.• Rays from closer objects are diverging and focus on a

point further than principal focus.• Biconcave lens causes the rays to diverge.• Larger the curvature greater the refractive power.• Refractive power is the reciprocal of focal length in

meters. • If the focal length is 25 cm, refractive power is 1/0.25= 4

diopters. Refractive power of human eye = 60 diopters

Jan. 2006 Eye- structure and optics 9

Refraction in Eye.

• When light rays from a distant object falls on the cornea, it is refracted:

• On entry into cornea,• On entry into aqueous humor,• On entry into lens, and• On entry into vitreous humor.• Finally an image is formed on the retina. The image is up

side down.

Jan. 2006 Eye- structure and optics 10

Accommodation.• Light rays from an object closer

than 6 meters will be diverging and the image will be formed behind the retina.

• In a camera, the lens is moved forwards to get the image on the film.

• In the eye, the lens curvature is increased to increase the refractive power and the image is formed on the retina.

• The reflex mechanism is accommodation.

• This can increase the refractive power by about 12 diopters.

Jan. 2006 Eye- structure and optics 11

Ciliary body and lens.

• Suspensory ligaments from the ciliary body keep lens in place, under tension when the ciliary muscles are at rest.

• The lens is pulled into a flattened shape by the tension because the lens substance is malleable and the lens capsule is elastic.

• Contraction of circular muscles reduce tension by sphincter like action and the meridional muscles reduce tension because they are attached to corneo-scleral junction and ciliary body.

• Reduction in tension results in increase in the curvature and refractive power of the lens.

Jan. 2006 Eye- structure and optics 12

Near Point.

• Light rays from very near to eye cannot be focused on the retina.

• The nearest point from which an object can be focused on the retina is the ‘near point’.

• Near point receds through out the life, 9 cm at 10 years, 10.5 at 20 years and 83 cm at 60 years.

• This is known as presbyopia and can be corrected by convex lens.

Jan. 2006 Eye- structure and optics 13

Refractive errors.

Jan. 2006 Eye- structure and optics 14

Astigmatism.

• When light rays go through lens with surfaces as part of sphere, the rays converge at the focal point.

• If light goes through cylindrical lens, the rays converge in a line parallel to the lens.

• When the surface of the cornea happens to be a part of distorted sphere, rays in different axis focus at different points resulting in blurred image.

• This can be identified by chart with radiating lines and corrected by cylindrical lens.

Jan. 2006 Eye- structure and optics 15

Iris and pupil.

• Iris is a pigmented, opaque membrane attached to ciliary body.

• Pupil is the opening at the centre if iris through which light rays reach the lens.

• The diameter of the pupil can be altered from 8 mm to pinpoint by the circular and radial muscles.

• The size of the pupil determines the amount of light reaching the retina.

• Smaller diameter increases the depth of focus and reduces spherical aberration of the periphery of the lens.