WAEC Physics 2011 Theory — Question 29
Question 29 of 39 from the West African Examinations Council (WAEC) Physics 2011 Theory paper, with the correct answer and a full explanation.
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13(d) The image of an object is located 9cm behind a convex mirror. If the magnification produced is 0.6, calculate the focal length of the mirror.
Model answer
Image distance v = -9cm (behind the mirror), magnification m = v/u = 0.6, so u = v/m = -9/0.6 = -15cm. Using 1/f = 1/u + 1/v: 1/f = 1/(-15) + 1/(-9) = -1/15 - 1/9 = -3/45-5/45 = -8/45? (Using magnitudes as per official solution): 1/f = 1/u - 1/v = 1/15 - 1/9 = (3-5)/45 = -2/45, therefore f = -22.5 cm.
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