Prisms in glasses are used to primarily correct double vision, for positional correction, or convergence correction. Recently, prisms have also been used to help people with hemianopia – a condition that causes blindness in half of the visual field in both eyes. This condition is common to those who have suffered brain injuries, and stroke victims.
If you suffer from the above issues, it can mean that an image one eye sees is not in line with what the other is seeing, thus creating an impression of two images instead of one. If glasses have a prism lens, it will correct the image, and will help to get the eyes working together as they should. It does this by using a prism to trick the brain into thinking that the eyes are working together, by shifting what you see slightly to align it with the other eye.
Normally, standard glasses only correct two types of eye conditions: focal distance (near or farsightedness) or an unusual curving of the cornea, such as astigmatism.
Prism glasses have a thinner apex and thicker base than standard glasses. This is to enable light to bend and slow. The angle, or index, of refraction that the light bends at will depend on the shape and thickness of the lens.
If you have started to get double vision, it is important you get it checked out as soon as possible by a qualified optician as this can be a symptom of wider problems such as diabetes, a brain tumour, high blood pressure, thyroid issues and cataracts.
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