Ray Optics and Optical Instruments - Result Question 34

«««< HEAD:content/english/neet-pyq-chapterwise/physics/ray-optics-and-optical-instruments/ray-optics-and-optical-instruments-result-question-34.md

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

======= ####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

3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed:content/english/neet-pyq-chapterwise/physics/ray-optics-and-optical-instruments/ray-optics-and-optical-instruments—result-question-34.md (a) $-10 cm$

(b) $20 cm$

(c) $-30 cm$

(d) $5 cm$

[2011M]

Show Answer

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

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

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