A lens of focal length 45 mm is used as a magnifier.The object being viewed is 6.8 mm long,and is positioned at the focal point of the lens.The lens is moved closer to the object,so that the image is now 25 cm from the lens.The distance the lens has been moved,in cm,is closest to:
A)0.69
B)0.76
C)0.62
D)0.55
E)0.48

Question

Quizplus · Accepted Answer

The lens formula is 1/f = 1/v - 1/u. Initially, the object is at the focal point, so u = f = 45 mm. When the image is 25 cm (250 mm) away, we use the lens formula to find the new object distance u'. Solving gives u' ≈ 38.1 mm. The lens has been moved by 45 mm - 38.1 mm = 6.9 mm, which is 0.69 cm.

A Lens of Focal Length 45 Mm Is Used as a Magnifier.The