Biology > Short and Long Sight
Two of the most common eye defects can be corrected using convex and concave lenses. To see clear crisp images, the light from an object must be focussed sharply onto the retina at the back of the eye. Usually the eye focuses by contracting or relaxing the ciliary muscles which in turn change the shape of the jelly-like lens.
Wearing glasses simply adds another lens in the light path, adjusting the focal length so that the image focuses onto the retina. Short sight can be corrected by wearing glasses with a suitable concave lens whereas long sight can be corrected by wearing a suitable convex lens.
Short sight (Myopia) is caused by the eye lens focussing light from distant objects short of the retina (see fig 1 below). It can also be caused by eyeball being too long. This causes far away objects to appear blurred.
Fig 1: Short sight (Myopia)
Placing a suitable concave lens in the light path before the eye lens enables the focal point of the light to be shifted, allowing the light from the distant object to focus nicely on the retina producing a crisp and focuses image.
Long sight (Hypermetropia) is caused by the eye lens focussing light from near objects at a point beyond the retina surface (see fig 2 below). It causes close objects to appear blurred. By placing a suitable convex lens in the light path before the eye lens, the focal points can be shifted, allowing the light from the near object to focus on the retina producing a focussed image.
Fig 2: Long sight (Hypermetropia)
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