Question

A child can see clearly only up to a distance of 25cm. He wants to read a book placed at a distance of 50 cm. Calculate the focal length of the lens.

Answer 1

Here,
Image distance, v=-25 cm,
Object distance, u=-50 cm
Using formula,
1/f=1/v -1/u
or 1/f=1/(-25) - 1/(-50)
or Focal length, f=-50 cm
Hence, a concave lens of focal length 50 cm should be used.