Ray Optics and Optical Instruments - Result Question 34

34. A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point $15 cm$ from the lens on the opposite side. If the lens is removed the point where the rays meet will move $5 cm$ closer to the lens. The focal length of the lens is

(a) $-10 cm$

(b) $20 cm$

(c) $-30 cm$

(d) $5 cm$

[2011M]

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Answer:

Correct Answer: 34. (c)

Solution:

  1. (c)

By lens formula,

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$u=10 cm$

$v=15 cm$

$f=$ ?

Putting the values, we get

$\frac{1}{15}-\frac{1}{10}=\frac{1}{f}$

$\frac{10-15}{150}=\frac{1}{f} \quad \therefore f=-\frac{150}{3}=-30 cm$



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