An object is placed 20 cm from a convex lens of focal length 15 cm. How far from the lens is the image?
Answer and explanation
Correct answer: A. 60 cm
- Step 1: Lens formula: 1/f = 1/u + 1/v (real-is-positive). Step 2. 1/v = 1/15 − 1/20, so v = 60 cm. Answer: 60 cm. Why not the others: 8.571 cm is what you get if you use 1/v = 1/f + 1/u. 5 cm is what you get if you subtract the distances. 35 cm is what you get if you add the distances.
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