UPSC 2014 CDS II GK Paper

The upper and lower portions in the common types of bifocal lensesare concave and convex, respectively. Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia,hypermetropia and/or astigmatism. Astigmatism is a type of refractiveerror in which the eye does not focus light evenly on the retina. Thisresults in distorted or blurred vision at all distances.