The focal length of a lens suggested to a person with Hypermetropia is 100cm. Find the distance of near point and power of the lens.

Explanation

Focal length of the lens, f = 100 cm
Let the distance of near point = d cm
Lens formula `(1)/(f) = (1)/(v)-(1)/(u)`
`(1)/(100)=(1)/(25)-(1)/(d) implies (1)/(d)=(1)/(25)-(1)/(100)=(4-1)/(100)=(3)/(100)`
`d=(100)/(3) = 33.33 cm`
(or)
`d=(25f)/(f-25) implies (25xx100)/(100-25) =(100)/(3) = 33.33 cm`
Power of lens, `P = (100)/(f("in cm")) =cancel (100)/cancel(100) = 1D`
`therefore` Power of lens = 1 Dioptre.