A short sighted person can see objects most distinctly of `16 cm`. If he wears spectacles at a distance of `1 cm` from the eye, what focal length should he have so as to enable him to see distinctly at a distance of `26 cm` ?

Explanation

As the person wears spectacles at a distance of `1 cm`. From the eyes.
`:. u = -(26 - 1)cm = -25 cm`,
and `v = -1(16 - 1) cm = -15 cm , f = ?`
As `(1)/(f) = (1)/(v)-(1)/(u)`
`:. (1)/(f)=(-1)/(15)+(1)/(25)=(-5 + 3)/(75)=-(2)/(75)`
`f = -(75)/(2) cm = -37.5 cm`.
`:.` A concave lens of focal length `37.5 cm`. is to be used.