Question

A candle placed 25 cm from a lens forms an image on a screen placed 75 cm on the other side 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 convex lens

Answer 1

Answer: (A)

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{1}{75}+\frac{1}{25}$
$\frac{1}{f}=\frac{100}{75\times 25}$
$f=\frac{75}{4}$
$=18.75cm$