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

Answer: (A)

• 18.75 cm and convex lens

Answer 2

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 $\frac { 1 } { f } = \frac { 1 } { v } - \frac { 1 } { u }$ ⇒ $\frac { 1 } { f } = \frac { 1 } { ( + 75 ) } - \frac { 1 } { ( - 25 ) }$

⇒ f = + 18.75 cm.