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Deprivation Amblyopia: This occurs when something obstructs light from entering the eye, preventing proper vision development. A congenital cataract is an example of such a condition. Strabismic Amblyopia: The most common form, this type arises when the eyes are not properly aligned, leading to double vision. To avoid confusion, the brain ...
When an eye focuses light correctly on to the retina when viewing distant objects, this is called emmetropia or being emmetropic. This means that the refractive power of the eye matches what is needed to focus parallel rays of light onto the retina. A distant object is defined as an object located beyond 6 meters (20 feet) from the eye.
The light and dark borders visible between color swatches do not exist (right). Corrective lenses bend the light entering the eye in a way that places a focused image accurately onto the retina. The power of any lens system can be expressed in diopters , the reciprocal of its focal length in meters.
Accommodation of the lens does not occur in emmetropia, and the lens is about 3.6 mm thick at the center; in accommodation, it thickens to about 4.5 mm. A relatively thin lens and relatively dilated pupil are also associated. The lens usually stiffens with age, causing less ability to focus when the eyes are not in a state of emmetropia. [3]
Many people, especially as they age, experience lightheadedness if they arise too quickly from a lying or seated position. Lightheadedness often accompanies the flu, hypoglycaemia, common cold, or allergies. Dizziness could be provoked by the use of antihistamine drugs, like levocetirizine, or by some antibiotics or SSRIs.
Argyll Robertson pupils (AR pupils) are bilateral small pupils that reduce in size on a near object (i.e., they accommodate), but do not constrict when exposed to bright light (i.e., they do not react). They are a highly specific sign of neurosyphilis; however, Argyll Robertson pupils may also be a sign of diabetic neuropathy.
This dilation may pose a problem since a larger pupil is less efficient at focusing light (see pupil, aperture, and optical aberration for more.) Patients who have accommodative spasm may benefit from being given glasses or contacts that account for the problem or by using vision therapy techniques to regain control of the accommodative system.
The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart. However, in any given moment of time, the eye can only sense a contrast ratio of ...