If a person can sees clearly at a distance of 100 cm, then find the power of leris used to see object at 40 cm.

Options

A. `-2.5` D and concave lens
B. `-2.5` D convex lens
C. `-3.5` D and concave lens
D. `-3.5` D and convex lens

Answer

A

Explanation

Here, in this case lens used by person should from the image of distant object at a distance if `40` cm in front of it
`:.mu=-oo, v=-40` cm
and `(1)/(f)=(1)/(v)-(1)/(u)` or, `(1)/(f)=(1)/(-40)-(1)/(-oo)`
or, `(1)/(f)=(1)/(-40)` or `f=-40` cm
` "Power" =(100)/(f)=(100)/(-40)=-2.5D`
Negative sign shows that lens used in concave lens.