What is meant by power of a lens? What is one dioptre?
Power of a lens is the reciprocal of its focal length. One dioptre is the power of the lens whose focal length is 1 m. It is also unit of Power.
A person needs a lens of power –5.5 dioptres for correcting his near vision and he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting?
(i) distant vision
(ii) near vision