The distance between an object and its real image in a convex lens is 40cm. If the magnification of the image is 3, calculate the focal length of the lens.

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A. 6.5 cm
B. 7.5 cm
C. 8.5 cm
D. 4.5 cm

Correct Answer: Option B

Explanation

u + v = 40

v/u = 3

v = 3u

u + 3u = 40

4u = 40

u = 10cm

v = 3u = 30cm

f = uv/(u+v)=10(30)/(10+30)=300/40

= 7.5 cm

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