Question

A real image $\frac{4}{5}$ times the size of the object is formed 18 cm from a convex lens. Then focal length of the lens is

• A. f = + 10 cm
• B. f = – 10 cm
• C. f = + 5 cm
• D. f = – 5 cm

Answer 1

The correct option is A f = + 10 cm

Given:
Image distance, v = +18 cm
Magnification, m = [as image is real]
According to the lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
Putting values in the formula we get,
⇒ f = +10 cm
Hence, the correct answer is option (a).