Question

what do you mean by power of lens how does it depend upon the focal length of lens what does positive and negative power represented please don't right unnecessary answer

Answer 1

power of lens is the reciprocal of focal length.
if power of lens increases focal length decreases.
therefore, if focal length is positive then power of lens is negative and when focal length is negative power of lens is positive.

Answer 2

power of a lens is defined as the measure of ability of a lens to converge or diverge the light beam to forn a diminiahed or magnified image
and power is also defined as reciprocal of focal length
P=1/f
=>Pis directly proportional to 1/f
sing concave lens is divergent the power is negative and convex lens is converging so the power is positive
another formula is also the P=1/f=[(n1-n2)/n1](1/R1-1/R2)
where n1 is refractive index of first medium and n2 is refractive index of second medium
R1 is radius of curvature of first surface of lens and R2 is the second radius of curvature of the lens.
the second formula is called lens makers formula.