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MCAT Question A Day - 9/7/13 - Answer!

What is the focal length of a lens that forms a virtual image 30 cm from the lens when a real object is placed 15 cm from the lens?

A.  10 cm
B.  15 cm
C.  30 cm
D.  45 cm

The correct answer is (C). The distance is given by the thin lens equation, 1/f = 1/p + 1/q. Since the image formed is virtual, the sign of q is negative.

Therefore: 1/f = 1/p - 1/q = 1/15 - 1/30 = 1/30 

f = 30 cm