Physics of Eyes and Vision 2024 PDF

physics of eyes and vision D R. E N T I D H A R A LTA E E COLLEGE OF MEDICINE M E D IC A L PH YSIC S Topics of the Lecture  Visual system  Vision Elements of the Eye  Focusing elements of the eye  Sensitivity of the eye Image formation on the retina  Image Formation on the Retina. Vision De...

physics of eyes and vision D R. E N T I D H A R A LTA E E COLLEGE OF MEDICINE M E D IC A L PH YSIC S Topics of the Lecture  Visual system  Vision Elements of the Eye  Focusing elements of the eye  Sensitivity of the eye Image formation on the retina  Image Formation on the Retina. Vision Defects Visual system The sense of vision consists of three major components: 1. The eyes that focus an image from outside world on the light sensitive retina. 2. The system of millions of nerves that carries the information deep into the brain. 3. The visual cortex-that part of the brain where, it is all put together. Blindness results if any one of the parts does not function Vision Elements of the Eye The cornea: It is the clear transparent bump on the front of the eye that does about two-third of focusing of light. Focusing elements of the eye The eye has two major focusing components: 1. The cornea is a fixed focus element. 2. The lens is variable in shape and has the ability to focus objects at various distances. Focusing elements of the eye The cornea focuses by bending (refracting) the light rays. The amount of bending depends on the curvatures of its surfaces and the speed of light in the lens compared with that in the surrounding material. The index of refraction is nearly constant for all corneas, but the curvature varies considerably from one person to another and is responsible for most our defective vision. Vision Elements of the Eye The Iris: It is the colored part of the front eye. The function of iris is adaptation of vision from light to dark and vice versa. It is believed that the iris aids the eye by increasing or decreasing incident light on the retina until the retina to adapted the new lighting condition.  In addition, under bright light conditions it plays an important role reducing lens defects. Vision Elements of the Eye The Pupil: It is the small opening in the center of iris where light enters the lens. It appears black because essential all of the light that enters is absorbed inside the eye.  (Under average light conditions, the opening is about ≈ 4mm). It can change from ≈ 3mm in diameter in bright light to 8mm in diameter in dim light. The physiologic reason form this change in size is not clear. Vision Elements of the Eye The lens: It is variable in shape and has the ability to focus objects at various distances at both its front and back surfaces. It is more curved in the back than in the front. The focusing power of lens is smaller than that of the cornea (1/3), because it is surrounded by substance that have indexes of refraction close to its own. The lens is made up of layers somewhat like onion and all layers do not have the same n. Vision Elements of the Eye The aqueous humor: It fills the space between the lens and the cornea. This fluid mostly waters, n ≈1.33. It is continuously being produced and the surplus escape through drain tube. “Canal of Schelemmt. Blockage of the drain tubes results in increased pressure in the eye. "Glaucoma". It maintains the internal pressure of the eye at about 20mm Hg. It contains many of the components of blood and provides nutrients to the non- vascularized cornea and lens. Vision Elements of the Eye The Vitreous humor: Is a clear jelly-like substance that fills the large space between the lens and the retina. It helps to keep the shape of the eye fixed and is essentially permanent. The sclera: Is the tough, white, light. tight covering over all of the eye except the cornea. The sclera is protected by a transparent coating called the conjunctive Vision Elements of the Eye The Retina: The light sensitive part of the eye. It converts the light image into electrical nerve impulses that one sent to the brain.  Sensitivity of the eye Image formation on the retina The retina-the light detector of the eye The retina, the light-sensitive part of the eye, converts the light images into electrical nerve impulses that are sent to the brain. The absorption of a light photon in a photoreceptor causes a photochemical reaction in it, which in some way initiates the action potential then produce electrical nerve impulses that are sent to the brain. Most vision is restricted to a small area called the macula lutea, or yellow spot. All detailed vision takes place in a very small area in the yellow spot (0.3mm in diameter) called the fovea centralis. Eye photoreceptors There are two general types of photoreceptors in the retina: the cones and the rods. The rods and cones are distributed symmetrically in all directions from the visual axis except in one region-the blind spot.  The cones The rods 6.5 million in each eye 120 million in each eye They are used for daylight, where we see fine details and They are used for night vision and for peripheral vision. recognize different colors. They primarily found in the fovea centralis They cover most of the retina but their maximum density at an angle of about 20° from the vision axis. Each of them has its nervous link to the brain. Hundreds of rods send their information to the same nerve fiber Their density determines the amount of details we can Their ability to resolve two close sources of light is poor. resolve in the eye. They have a maximum sensitivity at about 550 nm in the Most sensitive to the blue-green light (~510nm). yellow-green region. (The maximum solar spectrum of the earth surface). They are most rapid in dark adaptation (5min). They continue to dark adapt for 30 to 60 min. Image Formation on the Retina Diopter strength of the Eye The focal length F of a combination of two lenses with focal lengths F1 and F2 is given by Example1: Assume lens A with focal length FA=0.33m is combined with lens B with focal length FB=0.25m. What is the focal length of the combination?  Vision Defects Nearsightedness (Myopia) It is the ability to see near objects clearly, whereas distant objects are blurry. Due to the too strong eye lens or too long eye ball.  The eye over converges the nearly parallel rays from a distant object, and the rays cross in front of the retina. Vision Correction Myopia The correction for nearsightedness consists of placing a diverging eyeglass (concave) lens in front of the eye. Vision Defects Farsightedness (Hyperopia)  It is the ability to see far objects clearly, whereas distant objects are blurry.  Due to the too weak eye lens or too short eye ball.  A farsighted eye does not sufficiently converge the rays from a near object to make the rays meet on the retina. Vision Correction Hyperopia The correction for of far sightedness consists placing a converging eyeglass(convex) lens in front of the eye. Vision Defects Presbyopia It is the loss of the eye's ability to change its focus to see objects that are near. It is not a disease. It is part of the natural aging process of the eye, and can be easily corrected. Presbyopia generally starts to appear around age 45. Correct: It need Bifocal lens A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision. Vision Defects Astigmatism when astigmatism is present, point objects do not from clear point images on the retina this normally due to the corneas heaving unequal curvature in different directions Astigmatism is corrected with asymmetric lens Vision Defects HW HW1/ A myopic male has near and far point of 20cm and 250 cm respectively. What spectacle lens is prescribed for his defect and where is his near point. HW2/If a patient has a near point of 15cm without glasses and wears a correction lens of - 1.0 D, what his near point when wearing glasses?