Question Detail
Question
The focal length of a camera is 6 cm. By how much must the lens be moved to change from focusing on an object at a far distance to one 3 m away?
Correct Answer
Option A is the correct answer.
Detailed Explanation
For a far object, u = ∞, v = f = 6 cm. For an object at u = 3 m = 300 cm: 1/f = 1/u + 1/v => 1/6 = 1/300 + 1/v => 1/v = 1/6 - 1/300 = (50 - 1)/300 = 49/300 => v = 300/49 = 6.122 cm. The lens must be moved by 6.122 - 6 = 0.122 cm ≈ 0.12 cm.
Hint
Use the lens formula.