Question Detail
Question
A man who wears glasses of power 2.5 diopters must hold a newspaper at least 25 cm away to see the print clearly. How far away would the newspaper have to be if he took off the glasses and still wanted clear vision?
Correct Answer
Option D is the correct answer.
Detailed Explanation
The glasses form a virtual image at the near point. With glasses: u = -25 cm, f = 1/P = 1/2.5 = 0.4 m = 40 cm. 1/f = 1/v - 1/u => 1/40 = 1/v - 1/(-25) = 1/v + 1/25 => 1/v = 1/40 - 1/25 = (5-8)/200 = -3/200 => v = -200/3 = -66.7 cm. The near point without glasses is 66.7 cm.
Hint
Use the lens formula. The image formed by the glasses is at the person's near point.