A person needs a lens of power -5.5 dioptres for correcting his distant vision, for correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision and (ii) near vision ?

A person needs a lens of power -5.5 dioptres for correcting his distan

1.	A person needs a lens of power -5.5 dioptres for correcting his distant vision, for correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision and (ii) near vision ?
Answer» Solution :(i) For distant vision, given : Power of the LENS , `P_(1) = -5.5 D`. FOCAL length, `f = (1)/(P_(1)) = (1)/(-5.5) = 0.1873m = -18.73 cm` (II) For near vision, power is measured relative to the main PART of the lens. Therefore, `P_(1) + P_(2) = P` or `-5.5 + P_(2) = + 1.5` or `P_(2) = +7D` Focal length, `f = (1)/(P_(2)) = (1)/(+6.5)`