An object is placed 30 cm from a convex lens of focal length 10 cm. How far from the lens is the image?
Answer and explanation
Correct answer: B. 15 cm
- Step 1: Lens formula: 1/f = 1/u + 1/v (real-is-positive). Step 2. 1/v = 1/10 − 1/30, so v = 15 cm. Answer: 15 cm. Why not the others: 7.5 cm is what you get if you use 1/v = 1/f + 1/u. 20 cm is what you get if you subtract the distances. 40 cm is what you get if you add the distances.
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