Question Detail
Question
A simple microscope forms an image twice the size of the object. If the focal length of the lens of the microscope is 20 cm, how far is the object from the lens?
Correct Answer
Option A is the correct answer.
Detailed Explanation
For a simple microscope (magnifying glass), the magnification m = 1 + D/f. If m = 2 and D = 25 cm (near point), then 2 = 1 + 25/f => f = 25 cm. But the question gives f = 20 cm. Alternatively, for a simple microscope, m = v/u. If m = 2, v = 2u. Using the lens formula: 1/f = 1/u + 1/v = 1/u + 1/(2u) = 3/(2u). u = 3f/2 = 3 × 20/2 = 30 cm. The object distance is 30 cm.
Hint
Use m = v/u and the lens formula.