The eye behaves much as a pinhole camera. The image is inverted ,so that light falling nearest the nose (the nasal retina) comes from the lateral part of the visual field (the temporal field) while the more lateral part of the retina (the temporal retina) receives light from the central region of the visual field (the nasal field). Since objects may lie at different distances from the eye, the optical elements of the eye must be able to vary its focus in order to form a clear image on the retina. The eye of a young adult with normal vision are able to focus as distant as the stars and as close as 25 cm (the near point).

The optical power of the eye is at its minimum when distant objects are brought into focus and at its maximum when it is focused on the near point. This variation in optical power is achieved by the lens. As for any lens system, the optical power of the eye is measured in diopters. For a normal, relaxed eye the total optical power is about 58 diopters. Most of the optical power of the eye is provided by the refractive power of the cornea (about 43 diopters), while the lens contributes a further 15 diopters when the eye is focused on a distant object and about 30 diopters when it is focused on the near point. When the lens loses its elasticity, which commonly occurs with advancing age, the power of accommodation is reduced and the nearest point of focus recedes. This condition is known as presbyopia.

The ciliary muscles control the focusing of the eye: The lens is suspended from the ciliary muscle by the zonal fibers. When the ciliary muscles are relaxed there is a constant tension on the zonal fibers exerted by the effect of the intraocular pressure on the sclera. ihis tension stretches the lens and minimizes its curvature. Whenn the eye switches its focus from a distant to a near object, the ciliary muscle contracts and this opposes the tension in the sclera. As a result, the tension on the zonal fibers decreases and the lens is able to assume a more spheroidal shape due to its inherent elasticity. The more spheroidal the lens, the greater its optical power. The ciliary muscle is innervated by parasympathetic fibers from the ciliary ganglion.

The eye sees by abosbring the light rays refelcted of objects and converting these rays into elctonic signals, these are then sent to brain and interpreted as images. Light rays into the eye through the cornea and pupil then lens then focuses the image on to the retina's light sensitive cells, which record the image inverted and transmited to the brains visal center via the optic nerve.

The mechanism of seeing at night involves the sensitization of the rod cells by means of a pigment, called visual purple or rhodopsin, that is formed within the cells. Vitamin A is necessary for the production of visual purple; a deficiency of this vitamin leads to night blindness. Visual purple is bleached by the action of light and must be reformed by the rod cells under conditions of darkness. Hence a person who steps from sunlight into a darkened room cannot see until the pigment begins to form. When the pigment has formed and the eyes are sensitive to low levels of illumination, the eyes are said to be dark-adapted. A brownish pigment present in the outer layer of the retina serves to protect the cone cells of the retina from overexposure to light. If bright light strikes the retina, granules of this brown pigment migrate to the spaces around the cone cells, sheathing and screening them from the light. This action, called light adaptation, has the opposite effect to that of dark adaptation.

Source(s): All above information & images are based on information collected health.indiamart.com, from chapter on eyes from the book Human Physiology by Gillian Pocock and Christophor D. Richards and from various sources. All rights reserved by respective owners.

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Disclaimer: Any information displayed here is just for educational purposes, and may not be taken as an expert advice and should not be applied in life without consulting your eye doctor/specialist. We here by take no responsiblity of the accuracy of the above content as they have been taken from various sources.

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