Question

A candle placed $25cm$ form a lens forms an image on a screen placed $75cm$ on the other side of the lens. The focal length and type of the lens should be

• A. $+18.75cm$ and convex lens
• B. $-18.75cm$ and concave lens
• C. $+20.25cm$ and convex lens
• D. $-20.25cm$ and concave lens

Answer 1

Answer: (A)

$+18.75cm$ and convex lens

Given,

Object distance, $u=-25cm$

Image distance, $v=+75cm$

Focal length, $f$

Using lens formula,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\frac{1}{f}=\frac{1}{+75}-\frac{1}{-25}$

$=\frac{1+3}{75}$

$\frac{1}{f}=\frac{4}{75}$

$f=\frac{75}{4}=18.75cm$

The focal length is positive. So, it is a convex lens.