Question

3. A thin convex lens forms the distinct image of an
object on a screen placed at a distance of 30 cm from
the lens. When the lens is displaced 5 cm towards
the screen, then in order to obtain distinct image the
screen is displaced 5 cm towards the lens. Find the
focal length of the lens.
Ans. 10 cm.​

Answer 1

Explanation:

f=4DD2−Δ2

this is formula of finding focal length using displacement method.

f:− focal length

Δ:− displacement of lens done in order to get image again on screen.

D:−  distance between object and screen.

f=4×10102−22

f=2.4 cm

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Answer 2

Answer:

10cm

Explanation:

lens formula

and then equate the two equations

you will get a quadratic equation , one answer will be negative , and one will be positive

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