Question

The object and the image distances are 10 cm & 30 cm respectively for a lens. Image is formed on same side of the object then the focal length is .

• A. $25cm$
• B. $-10cm$
• C. $+15cm$
• D. $-15cm$

Answer 1

Answer: (C), (D)

$-15cm$

Given below
Image distance $v=-30cm$, (using the sign conventions)
Object distance $u$ = $-10cm$ (using the sign conventions)

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{-30}-\frac{1}{-10}=\frac{1}{\text{f}}$
$f=15\text{cm}$
Here, positive sign shows that the lens is convex.