Has your eye doctor prescribed prism eyeglasses for you or your child in a new eyeglass prescription? It could be due to crossed eyes, lazy eye, or some diseases of the eyes or body.
Prisms are thin pieces of the optical material that is used in prescription eyeglasses. You may remember them from high school physics. They have a base that is thicker and an apex that is thinner. Due to the light bending properties of the lens material, the thicker lens base slows light down as it passes through. Since the prism is thicker at the bottom base it slows light down longer than the top apex, and light changes direction down towards the base as it exits the prism. If the lens were equally thick throughout, it would slow the light down but the direction would remain unchanged. The amount of direction change is determined by the index of refraction of the material compared to the index of refraction of air. More dense materials allow optometrists to make the ultra thin lenses that have drastically improved eye comfort in recent years.
Eyes that cross can turn in or out, and also one eye can turn up or down. A combination of eyes turning laterally and vertically is common. When this results in double vision, it can be completely disruptive to mobility and lifestyle of the affected person. If the eye turns are present at birth, there may be no double vision present. The brain has the capacity to suppress or turn off the area of vision that results in the eyes perceiving double at early ages. When a person sees double, prisms and surgery are the two options eye doctors have to try and restore normal visual functioning. Prisms do not appear as a strange looking triangle in the lens. They usually show up as thicker and thinner edges on the eyeglass lenses. Normally they are ground into the shape of the lenses, but because of optical properties, some lenses can have the optical center repositioned to induce prism. There can be an adjustment to prism added to a prescription while the brain relearns how to interpret the eyes seeing single. When diseases such as strokes and diabetes cause double vision there can be some fluctuation over time, and frequent eyeglass prescriptions changes to adjust the amount of prism may be required.
Your eye doctor may prescribe prism in your eyeglasses for other reasons. Some people will only have a tendency for an eye to turn, and while it may not actually turn it will result in eyestrain, fatigue with reading, headaches from using your eyes, and other symptoms. In cases of traumatic head injuries yoked prisms may be used to help retrain a disrupted visual system.
Some serious health problems can cause double vision, and any new onset or increasing condition of double vision should be thoroughly examined by your optometrist. When double vision occurs after the age of fifty, common causes are thyroid conditions, high blood pressure and diabetes. Try to notice when it occurs and what makes it worse to help your optometrist in treating your eye condition with the best methods possible. In the case of a brain tumor inducing double vision, a trip to the eye doctor could save your life.