\nVision defects generally come in two different ways called short sightedness and long sightedness. A person who has short sight can see near objects clearly, but struggles to focus on distant ones. This is caused by one or two reasons....\nVision defects generally come in two different ways called short sightedness and long sightedness. A person who has short sight can see near objects clearly, but struggles to focus on distant ones. This is caused by one or two reasons. Firstly the eyeball could be elongated such that the light gets focused in front of the retina which produces a blurry image. Another reason is due to the lens being too powerful, again, resulting in the light converging to a point in front of the retina. The way to correct short sightedness is to place a diverging lens in front of the eye, in the form of glasses of course. This spreads the light out slightly before it reaches the eyes so the light gets focused properly inside it. For someone with long sight, the opposite occurs, they can see distant objects clearly but their near point is further than someone with good vision. The reasons are due to the eyeball being too short which ends up focusing the light behind the retina. Or the lens is too weak to converge the light properly so the focal point is also behind the retina. This is often a result of ageing, which is why you usually need glasses when you get older. The lens used in this case is a converging lens. Obviously different people will have different amounts of vision defects, which is where the power of the lens is important. The power of a lens can be calculated by using the simple equation P=1/f. Where P is the power in diopters and f is the focal length in metres. The power of a diverging lens is always negative since the focal point is in front of the lens so is measured as a negative value. For example a diverging lens with a focal length of -0.15m has a power of -6.67 diopters. If glasses aren’t for you then you can always go for the permanent option of laser eye surgery to fix your vision defect. This works by the laser cutting into the cornea and reshaping it, such that the light focuses directly onto the retina. It apparently doesn’t hurt during the procedure but I can’t imagine it to be the most comfortable thing either!\n
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