Question

# An object of height 4.25 mm is placed at a distance of 10 cm from a convex lens of power +5 D. Find (i) focal length of the lens, and (ii) size of the image.

Solution

## Object height, h = 4.25 mm = 0.425 cm Object distance, u = -10 cm Power, P = +5 D Focal length, f = ? Image distance, v = ? Image height, h' =? Power, P=1ff=1P=15=0.2 m=20 cm Using the lens formula, we get: 1v−1u=1f⇒1v−1−10=120⇒1v+110=120⇒1v=120−110⇒1v=−120v=−20 cm Now magnification, m=h′h=vu Substituting the values we get: h′0.425=−20−10h′=2×0.425=0.85 cm=8.5 mm Thus the image is 8.5 mm long; It is also erect and virtual. PhysicsScience for Tenth Class - Part I - PhysicsStandard X

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