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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:

1v1u=1f1v110=1201v+110=1201v=1201101v=120v=20 cm

Now magnification, m=hh=vu

Substituting the values we get:

h0.425=2010h=2×0.425=0.85 cm=8.5 mm

Thus the image is 8.5 mm long; It is also erect and virtual.


Physics
Science for Tenth Class - Part I - Physics
Standard X

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