JEE Main 2014 · 6 Apr · Shift 1 · Physics Q23

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PhysicsSingle correct+4 / -1Question 23 of 90

A thin convex lens made from crown glass $\left(\mu=\dfrac32\right)$ has focal length $f$ . When it is measured in two different liquids having refractive indices $\dfrac43$ and $\dfrac53$ , it has the focal lengths $f_1$ and $f_2$ respectively. The correct relation between the focal lengths is :

f_1=f_2<f

f_1>f

and

f_2

becomes negative

f_2>f

and

f_1

becomes negative

f_1

and

f_2

both become negative

Answer and worked solution

Official answer: B

Lens-maker: $\dfrac1f\propto\left(\dfrac{\mu}{\mu_l}-1\right)$ .

In air: $0.5$ .
In $\mu_l=\frac43$ : $\frac{1.5}{4/3}-1=0.125$ , so $f_1=4f>f$ .
In $\mu_l=\frac53$ : $\frac{1.5}{5/3}-1=-0.1$ , so $f_2$ is negative (a diverging lens).

Topics:Thin lenses and lens maker's formula

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JEE Main 2014 · 6 Apr · Shift 1, Physics question 23