Question

information about the lens available in an optical shop.find out how the focal length of a lens may be determine by the given 'power' of the lens

Answer 1

    There are usually Biconcave and plano-concave lenses, Biconvex and Plano-convex lenses and cylindrical lenses available in the optical shops.  There are some other lenses too available.

    The convex lenses correct short sight.  The concave lenses correct long sight problem.  The cylindrical lenses correct distortion of objects due to astigmatism.   There are other types of spectacles which do correct color defects and others...

   Power of a lens P = 1/f     where f = focal length in meters.
 
   so f = 1/P     where P is expressed in Diopters.