F o u r b as i c p r o c e s s ar e i n vo l ve d i n t h e f o r mat i o n o f an i mag e o n r e t i n a Refraction of light rays Accommodation of lens Constriction of pupil Convergence
Ref r ac t i on of Li g ht Ray s• When light rays traveling through a transparent substance pass into a second transparent substance with a different density, they bend at the junction between the two substances. This bending is called refraction.
The lens system of the eye is composed of four refractiveinterfaces:(1) the interface between air and the anterior surface of thecornea,(2) the interface between the posterior surface of the cornea andthe aqueous humor,(3) the interface between the aqueous humor and the anteriorsurface of the lens of the eye,(4) the interface between the posterior surface of the lens andthe vitreous humor.
Ac c om odat i on of l ens mConvex lens will refract incoming light rays toward each other, so that theyeventually intersect.Concave lens causes light rays to refract away from each other.
The lens of the eye is convex on bothits anterior and posterior surfacesIts focusing power increases as itscurvature becomes greater. When the eye is focusing on a closeobject, the lens becomes more curved,causing greater refraction of the lightrays.This increase in the curvature of thelens for near vision is calledaccommodation.
Ho w d o e s ac c o mmo d at i o n o c c u r ?viewing distant objects –the ciliary muscle of the ciliary body is relaxed –the lens is flatter [ because it is stretched in all directions by zonularfibers. ]viewing a close object,-the ciliary muscle contracts –releases tension on the lens and zonular fibers. –the lens becomes more spherical (more convex) –which increases its focusing power and causes greater convergence ofthe light rays. Parasympathetic fibers of the oculomotor (III) nerve innervate the ciliarymuscle of the ciliary body and, therefore, mediate the process ofaccommodation
Cons t r i c t i on of pupi lThe pupil of the human eye can become as small as about 1.5 mm and as largeas 8 mm in diameter..The constriction of the pupil is a narrowing of the diameter of the holethrough which light enters the eye due to the contraction of the circularmuscles of the iris.This autonomic reflex occurs simultaneously with accommodation and preventslight rays from entering the eye through the periphery of the lens.Light rays entering at the periphery would not be brought to focus on the retinaand would result in blurred vision.The greatest possible depth of focus occurs when the pupil is extremely small. -with a very small aperture, almost all the rays pass through the center ofthe lens, and the central most rays are always in focus.
Conver genc eIn humans, both eyes focus on only one set of objects—acharacteristic called binocular vision.B.V allows the perception of depth and an appreciation of the 3 Dnature of objects.B.V occurs when light rays from an object strike correspondingpoints on the two retinas. When we stare at a distant object, the incoming light rays are aimeddirectly at both pupils and are refracted to identical spots on theretinas of both eyes.As we move closer to an object, however, the eyes must rotatemedially for the light rays from the object to strike the same pointson both retinas.
• The term convergence refers to this medial movement of the two eyeballs so that both are directed toward the object being viewed.• The nearer the object, the greater the degree of convergence• The coordinated action of the extrinsic eye muscles brings about convergence. convergence
F o r mat i o n o f i mag e o n r e t i n a• The image on retina is inverted and reversed with respect to the object.• However mind perceives objects in the upright position because the brain is trained to consider an inverted image as the normal.
• photochemicals in cones are similar to rhodopsin (scotopsin + retinal)• cones contain photopsin + retinal• 3 different types of photochemicals are present in cones, their light absorption spectra are different cone pigment wavelength of peak absorption (nm) blue-sensitive pigment 445 green-sensitive pigment 535 red-sensitive pigment 570• rods have peak sensitivity at 505 nm
• Human eye can see any colour due to a combination of red, green and blue monochromatic light in different proportions.