JEE Advanced 2013 · Paper 1 · Physics Q10

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PhysicsSingle correct+2Question 10 of 60

The image of an object, formed by a plano-convex lens at a distance of 8 m behind the lens, is real and is one-third the size of the object. The wavelength of light inside the lens is $\dfrac23$ times the wavelength in free space. The radius of the curved surface of the lens is

1 m

2 m

3 m

6 m

Answer and worked solution

Official answer: C

Refractive index. $\mu=\dfrac{\lambda_0}{\lambda}=\dfrac32$ .

Focal length. Real image one-third size: $v=8$ m, $u=-24$ m. $\dfrac1f=\dfrac18+\dfrac1{24}=\dfrac16$ , so $f=6$ m.

Plano-convex lens. $\dfrac1f=\dfrac{\mu-1}R$ , so $R=0.5\times6=3$ m.

(C)

Topics:Thin lenses and lens maker's formula

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JEE Advanced 2013 · Paper 1, Physics question 10