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The human eye is a complex mechanism
in many ways similar to a camera. Just as the camera records images
on a film, the eye records them on a special layer called the retina.
Just like the camera needs lenses to focus images on the film, the
eye also has lenses to focus images on the retina. The eye has two
lenses: the cornea and the "lens". The cornea is a fixed
focus lens. It simply bends light rays by a fixed amount. The lens
is like an auto focus system. It changes its shape to focus images
perfectly on the retina, so that images appear sharp and crisp.
Sometimes when there is a mismatch between the curvature of the
cornea, the power of the lens and the length of the eye, the eye
focuses images ahead of or behind the retina. This is known as a
refractive error. This results in unclear vision, eyestrain and
headaches. To eliminate this mismatch, spectacles or contact lenses
need to be worn.
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