Dr. Jie ZouPHY The Human Eye Fundamental elements of an eye: –Cornea: light enters the eye through the transparent outer coating of the eye. –Aqueous humor –The adjustable lens –The jellylike vitreous humor –Light-sensitive retina The retina is covered with millions of small structures known as rods and cones. –Optical nerve

Dr. Jie ZouPHY Image Production in the Eye Light enters the eye through the cornea and the lens. It is focused onto the retina. The eye produces a real, inverted image on the retina.

Dr. Jie ZouPHY Optical Properties of the Eye Most of the refraction needed to produce an image occurs at the cornea. –The reason is that the difference in index of refraction is greater at the air-cornea interface than at any interface within the eye. –n (air) ≈ 1.00, n(cornea) ≈ 1.38, n(aqueous humor) = 1.33, n(lens) = 1.40, n(vitreous humor) = By altering the shape of the lens with the ciliary muscles, we are able to change the precise amount of refraction the lens produces, which, in turn, changes its focal length.

Dr. Jie ZouPHY Accommodation in the Human Eye Accommodation: The process of changing the shape of the lens, and hence adjusting its focal length, is referred to as accommodation. –When the eye is viewing a distant object, the ciliary muscles are relaxed and the focal length of the lens is at its greatest. –When the eye is focusing on a near object, the ciliary muscles are tensed, changing the shape and reducing the focal length of the lens.

Dr. Jie ZouPHY Near Point and Far Point The lens can be distorted only so much. Hence, there is a limit to how close the eye can focus. An object that is closer to the eye than its near point appears fuzzy no matter how hard we try to focus on it. –For young people, typically, the near point distance N = 25 cm. –Persons 40 years of age, N may be 40 cm from the eye. –In later years, the near point distance N may move to 500 cm or more (eventually, reading glasses may be necessary). Far point: The greatest distance an object can be from the eye and still be in focus. –Normal far point is essentially infinity.

Dr. Jie ZouPHY Iris and Pupil The amount of light that reaches the retina is controlled by a colored diaphragm, called the iris. As the iris expands or contracts it adjusts the size of the pupil: the opening through which light enters the eye. –The iris dilates the pupil in dim light and contracts it in bright light.

Dr. Jie ZouPHY Nearsightedness Normal vision: –Can focus on objects at infinity when the ciliary muscles of the eye relax. –The far point is essentially infinity. Nearsightedness: –A totally relaxed eye focuses only out to a finite distance from the eye. –The far point is of a finite distance from the eye. –A person with this condition is said to be nearsightedness because objects near the eye can be focused, while objects beyond the far point are fuzzy.

Dr. Jie ZouPHY Eye Shape and Nearsightedness An eye that is elongated can cause nearsightedness. The eye converges the light coming into it in too short a distance-or in other words, the focal length of the eye is less than the distance from the lens to the retina. An object at infinity forms an image in front of the retina.

Dr. Jie ZouPHY Correcting Nearsightedness To correct this condition, we need to “undo” some of the excess convergence produced by the eye, so that images again fall on the retina. A diverging lens in front of the eye can correct for nearsightedness. For an object far away, a diverging (concave) lens with proper focal length produces a virtual image of the object at the nearsighted person’s far point. The person’s relaxed eye can now focus on the image of the object. Use a diverging lens in front of the eye to “undo” some of the excess convergence.

Dr. Jie ZouPHY Farsightedness Normal vision: –Near point distance  25 cm. Farsightedness: –The vision of a farsighted person differs from that of a person with normal vision by having a near point that is much farther from the eye than the usual 25 cm. –As a result, a person who is farsighted can see clearly beyond a certain distance-the near point-but cannot focus on closer objects.

Dr. Jie ZouPHY Eye Shape and Farsightedness Farsightedness can be caused by an eyeball that is shorter than normal, or by a lens that becomes sufficiently stiff with age that it can no longer take on the shape required to focus on nearby objects. An object inside the near point comes to a focus behind the retina.

Dr. Jie ZouPHY Correcting Farsightedness A converging (convex) lens in front of the eye can correct for farsightedness. The convex lens focuses light from an object inside the the near point to produce an image that is beyond the near point. The eye can now focus on the image of the object. Using a converging lens in front of the eye to add to its insufficient convergence.

Dr. Jie ZouPHY Refractive Power Refractive power: the ability of a lens to refract light. It is related to its focal length. Refractive power = 1/f –The shorter the focal length, the more strongly a lens refracts light. –Refractive power depends inversely on the focal length. –SI unit of refractive power: diopter = m -1.