Question

A convex lens of focal length 10 cm forms an image 15 cm from the lens. How far is the object from the lens?

• A. 20 cm
• B. 30 cm
• C. 40 cm
• D. 60 cm

Answer 1

The correct option is B 30 cm

Since it is a convex lens,
f = 10 cm
From the question,
v = 15 cm
Using the lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
Putting the values of $v\text{and}f$,
$\Rightarrow \frac{1}{15}-\frac{1}{u}=\frac{1}{10}$
$\Rightarrow \frac{1}{15}-\frac{1}{10}=\frac{1}{u}$
$\Rightarrow -\frac{1}{30}=\frac{1}{u}$
$\Rightarrow u=-30\text{cm}$
Hence, the distance of the object from the lens is 30 cm.