An object is placed 15 cm in front of a convex (converging) lens of focal length 10 cm. What are the image distance and image characteristics?
Answer and explanation
Correct answer: D. 30 cm behind the lens; real, inverted, and magnified
- Step 1: Thin lens equation: 1/f = 1/d_o + 1/d_i. Step 2. 1/10 = 1/15 + 1/d_i => 1/d_i = 1/10 - 1/15 = 3/30 - 2/30 = 1/30. Step 3. d_i = +30 cm (positive means real image on opposite side). Magnification m = -d_i / d_o = -30 / 15 = -2 (inverted and magnified). Answer: 30 cm behind the lens; real, inverted, and magnified. Why not the others: Virtual images have negative image distances.
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