How can we determine the focal length and power of the convex lens required to correct a hypermetropia eye ?

How can we determine the focal length and power of the convex lens req

1.	How can we determine the focal length and power of the convex lens required to correct a hypermetropia eye ?
Answer» Solution : Let X = distance of the near point N. from the hypermetropic eye. We know that the near point N of the NORMAL eye is at distance D = 25 cm. The object placed at N forms its virtual image at N. due to the convex lens. So, object distance = u = -D,Image distance = v = -x. focal length = f = ? By lens FORMULA, `(1)/(f) = (1)/(v) - (1)/(u) = (1)/(-x)-(1)/(-D) = (x-D)/(xD)` `:.` required focal length, Required Power , `P = (1)/(f) = (x-D)/(xD)` As `x gt D` both f and D are positive. Thus, the correcting lens must be a convex lens.