Physiology of Vision Optics of Eye Dr. Sumera Gul

Learning Objectives At the end of the lecture the students should be able to: Explain the basic physiological anatomy of eye Discuss the refractive index, refractive power and refractive principles to lenses applicable in eye Explain focal length and focal point Discuss the effects of various types of lenses on refraction of light rays

The Eye

Protectors of the Eye Boney Orbit of the eye Exto-ocular muscles Eye lids Eye lashes Conjunctiva Tears (enzyme lysosyme inhibits bacterial growth)

The Structure of the Eyeball The Sclera, - thick fibrous outer coat which gives resistance and durability to the eye. The Choriod - delicate vascular coat which supplies nourishment to the inner layer of the retina. The Retina - photosensitive layer of the eye which translates the light impulses into electronic impulse which travels down the optic pathways to the brain.

Cornea Transparent avascular tissue with 5 distinct layers. Function is to allow light rays to enter the eye. Surrounded by a vascular layer known as the limbus which nourishes the cornea. Should be smooth and clear.

Tears Normal Tears Reflect Tears Lubricant Protect from bacterial infections Contain an enzyme called lysozyme Reflect Tears Mostly water Back-up for irritation, emotions and dryness

Lens The lens capsule is a biconvex, avascular, colorless almost clear structure About 65% water and 35% protein Function is to help focus light waves onto the retina by changing the curvature of the lens No pain fibers, blood vessels or nerves in the lens.

Retina Thin semitransparent multi-layered sheet of neural tissue that lines the inner layer of the posterior section of the globe of the eye.

Refractive Index

Refractive Index Light rays travel through air at a velocity of about 300,000 km/sec. Velocity of light rays is much slower through transparent solids and liquids. The ratio of the velocity of light in air to the velocity in the substance. The refractive index of air is 1.00.

Refractive Index If light travels through a particular type of glass at a velocity of 200,000 km/sec. What will be the refractive index of this glass?

Refractive Index If light travels through a particular type of glass at a velocity of 200,000 km/sec. What will be the refractive index of this glass? Refractive index of a substance = The velocity of light in air The velocity of light in the substance.

Refractive Index If light travels through a particular type of glass at a velocity of 200,000 km/sec. What will be the refractive index of this glass? Refractive index of a substance = The velocity of light in air The velocity of light in the substance. = 300,000 = 1.50 200,000

What happens when light rays traveling forward in a beam strike an interface that is perpendicular to the beam?

The rays enter the second medium without deviating from their course. The only effect that occurs is decreased velocity of transmission and shorter wavelength

What happens when light rays traveling forward in a beam strike an interface that is angulated to the beam?

Refraction This bending of light rays at an angulated interface is known as refraction. Note particularly that the degree of refraction increases as a function of (1) the ratio of the two refractive indices of the two transparent media and (2) the degree of angulation between the interface and the entering wave front.

Convex Lens Focuses Light Rays. focal point

Concave Lens Diverges Light Rays.

Focal Point The distance beyond a convex lens at which parallel rays converge to a common focal point is called the focal length of the lens.

Refractive Power

The more a lens bends light rays, the greater is its “refractive power This refractive power is measured in terms of diopters.

The refractive power in diopters of a convex lens is equal to 1 meter divided by its focal length. Thus,a spherical lens that converges parallel light rays to a focal point 1 meter beyond the lens has a refractive power of +1 diopter. If the lens is capable of bending parallel light rays twice as much as a lens with a power of +1 diopter, it is said to have a strength of +2 diopters, and the light rays come to a focal point 0.5 meter beyond the lens.

The lens system of the eye is composed of four refractive interfaces: (1) the interface between air and the anterior surface of the cornea, (2) the interface between the posterior surface of the cornea and the aqueous humor, (3) the interface between the aqueous humor and the anterior surface of the lens of the eye, (4) the interface between the posterior surface of the lens and the vitreous humor.