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Question

A biconvex lens has a radius of curvature of magnitude 20 cm and the refractive index of the lens is 1.5. Which of the following option describe best the image formed of an object of height 2 cm placed 30 cm from the lens?

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Solution

Sign convention: Direction along incident ray is taken as +ve.
For the given biconvex lens,

Distance of object from lens, u=30 cm
R1=+20 cm, R2=20 cm
Hence, by using lens maker's formula
1f=(μ2μ11)(1R11R2)
1f=(μg11)(1201(20))
or, 1f=(1.51)(220)
f=+20 cm
We know,
m=ff+u
m=202030=2
Also, m=Height of image (hi)Height of object (ho)
hi(+2 cm)=2
hi=4 cm (real and inverted)

Why this question ?Tip: Negative sign of height of image represents that image is real, inverted. Thus image has formed below principle axis..

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