Question

the lenses available in an optical shop.find out how the focal length of a lens may be determined by the given "power" of the lens.

Answer 1

The power of a lens is measured in diopters(D) and is expressed as the inverse of the focal length. 

D= 1/f

Therefore focal length is also the inverse of the power of the lens. This means  that if power of a lens is 2, the focal length will be 1/2 a meter.

Answer 2

    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.

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