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