Refractive Power and Accommodation PDF

Refractive power is greatest when the curvature of a lens is greatest. The refractive power of a lens is measured in diopters, the number of diopters being the reciprocal of the principal focal distance in meters. For example, a lens with a principal focal distance of.25m has a refractive power of 1...

Refractive power is greatest when the curvature of a lens is greatest. The refractive power of a lens is measured in diopters, the number of diopters being the reciprocal of the principal focal distance in meters. For example, a lens with a principal focal distance of.25m has a refractive power of 1/.25, or 4 diopters. The human eye has a refractive power of 60 diopters at rest In the eye, light is refracted at the anterior surface of the cornea and at the anterior and posterior surfaces of the lens. The process of refraction can be presented diagrammatically, however, without introducing any appreciable error, by drawing the rays of light as if all refraction occurs at the anterior surface of the cornea Note that the retinal image is inverted. The connections of the retinal receptors are such that from birth, any inverted image on the retina is viewed right side up and projected to the visual field on the opposite side to the retinal area stimulated This perception is present in infants and is innate If retinal images are turned right side up by means of special lens, the objects viewed look as if they are upside down Normal sight→ faraway object is clear Myopia → eyeball too long Hyperopia → eyeball too short Strabismus and Amblyopia ○ Strabismus is a misalignment of the eyes and one of the most common eye problems in children, affecting 4% of children under 6 years of age. It is characterized by one or both eyes turning inward (esotropia), outward (exotropia), upwards, or downward. In some cases, more than one of these conditions is present. Strabismus is commonly called “wandering eye” or “crossed eye” ○ It results in visual images that do not fall on corresponding retinal points. ○ When the visual images chronically fall on non corresponding points in the two retina in children, one is eventually suppressed (suppression scotoma). This suppression is a cortical phenomenon, and rarely develops in adults ○ It is important to institute treatment before age 6 in affected children, because if the suppression persists, the loss of visual acuity in the eye generating the suppressed image is permanent. ○ A similar suppression with subsequent permanent loss of visual acuity can occur in children in who vision in one eye is blurred or distorted due to refractive error ○ The loss of vision in these cases is called amblyopia ex anopsia, a term that refers to uncorrectable loss of visual acuity that is not directly due to organic disease of the eye. Typically an affected child has one weak eye with poor vision and one strong eye with normal vision. It affects about 3% of the general population ○ Amblyopia is also referred to as “lazy eye”, and it often coexists with strabismus Accomodation ○ When the ciliary muscle is relaxed, parallel light rays striking the optically normal (emmetropic) eye are brought to a focus on the retina ○ as long as this relaxation is maintained, rays from objects closer than 6 m from The Observer are brought to a focus behind the retina and consequently the objects appear blurred ○ The problem of bringing diverging rays from close objects to a focus on the retina can be solved by increasing the distance between the lens and the retina or by increasing the curvature or refractive power of the lens ○ The process by which the The curvature of the lens is increased is called accommodation. At rest, the lens is held under tension by the lens ligaments. Because the lens substance is malleable and the lens capsule has considerable elasticity, the lens is pulled into a flattened shape. if the Gaze is directed at a near object, the ciliary muscle contracts. This decreases the distance between the edges of the ciliary body and relaxes the lens ligaments, so that the lens Springs into a more convex shape. The change is greatest in the anterior surface of the lens. ○ In young individuals, the change in shape may add as many as 12 diopters to the refractive power of the eye. The relaxation of the lens ligaments produced by contraction of the ciliary muscle is due partly to the sphincter-like action of these circular muscle fibers in the ciliary body and partly to the contraction of longitudinal muscle fibers that attach anteriorly, near the corneal Junction. ○ As these fibers contract, they pull the whole ciliary body forward and inward. this motion brings the edges of the ciliary body closer together. the nearest point to the eye at which an object can be brought into clear focus by accommodation is called the near point division. ○ The near point recedes throughout life, slowly at first and then rapidly with advancing age, from approximately 9 cm at age 10 to approximately 83 cm at age 60. This recession is due principally to increasing hardness of the lens, with the resulting loss of accommodation due to the steady decrease in the degree to which the curvature of the lens can be increased. By the time a healthy individual reaches age 40 to 45 years, the loss of accommodation is usually sufficient to make reading and close work difficult. This condition, which is known as presbyopia, can be corrected by wearing glasses with convex lenses. In addition to accommodation, the visual axes converge, and the pupil constricts when an individual looks at near objects. This three-part response is called the near response Rhodopsin ○ Rhodopsin (visual purple) is the photosensitive pigment in the rods and is composed of retinal, an aldehyde of vitamin A, and the protein opsin. Vitamin A is needed for the synthesis of retinal, so a deficiency in this vitamin produces visual abnormalities Dark adaptation ○ If a person spends a considerable length of time in brightly lit surroundings and then moves to a dimly lit environment, the retinas slowly become more sensitive to light as the individual becomes accustomed to the dark. ○ This decline in visual threshold is known as dark adaptation. it is nearly maximal in about 20 minutes although some further decline occurs over longer periods. the time required for dark adaptation depends in part on the time needed to build up the rhodopsin stores that are continuously being broken down in bright light. ○ When one passes suddenly from a dim to a brightly lit environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises. ○ this adaptation occurs over a period of about 5 minutes and is called light adaptation, although it is merely The Disappearance of dark adaptation ○ The dark adaptation response has two components. the first drop in visual threshold, rapid but small and magnitude, is due to dark adaptation of the cones because when only the foveal, rod free portion of the retina is tested, the decline proceeds no further. in the peripheral portions of the retina, a further drop occurs because of adaptation of the rods. ○ the total change in threshold between the light adapted And the fully dark adapted eye is very great ○ radiologists, aircraft pilots, and others who need maximum visual sensitivity and dim light can avoid having to wait 20 minutes in the dark to become dark if they were red goggles when in bright light. ○ light wavelengths in the red end of the spectrum stimulate the rods to only a slight degree while permitting the cones to function reasonably well therefore a person wearing red glasses can see in bright light during the time it takes for the rods to become dark adapted Color vision ○ Colors have 3 attributes: hue, intensity, and saturation (degree of freedom from dilution with white). ○ For any color there is a compensatory color that, when properly mixed with it, produces a sensation of white ○ Black Is the sensation produced by the absence of light, his but it is probably a positive sensation because the Blind Eye does not see black rather it sees nothing ○ another observation of basic importance is a demonstration that the sensation of white, any spectral color, and even the extra spectral color, purple, can be produced by mixing various proportions of red light (wavelength 723-647), green light (wavelength 575-492) and blue light (492-450). ○ Red, green and blue are therefore called the primary colors. A third important point is that the color perceived depends in part on the color of the other objects in the visual field. ○ thus for example a red object is seen as red if the field is illuminated with green or blue light, but as pale pink or white if the field is eliminated with red light