Question

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

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

Answer 1

The correct option is A + 18.75 cm and convex lens

In concave lens, image is always formed on the same side of the object. Hence the given lens is a convex lens for which u = – 25 cm, v = 75 cm.
By using $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{(+75)}-\frac{1}{(-25)}\Rightarrow f=18.75cm.$