NTSE Stage 2 SAT 2018 Exam Question Paper

Question : 36 of 100

Marks: +1, -0

Two convex lenses A and B each of focal length 30 cm are separated by 30 cm, as shown in the figure. An object O is placed at a distance of 40 cm to the left of lens A.

What is the distance of the final image formed by this lens system?

120 cm to right of lens A
90 cm to right of lens A
22.5 cm to right of lens B
45 cm to right of lens B

Validate

Solution:

For lens A,

\;\;{1}/{f}=\;\;{1}/{v}-\;\;{1}/{u}

⇒\;\;{1}/{30}=\;\;{1}/{v}-(-\;\;{1}/{40})

⇒1/30=1/v+1/40

=\;\;{40-30}/{1200}=\;\;{10}/{1200}

or v = 120cm
For lens B,u = 90cm

\;\;{1}/{f}=\;\;{1}/{v}-\;\;{1}/{u}⇒\;\;{1}/{30}=\;\;{1}/{v}-\;\;{1}/{90}

\;\;{1}/{v}=\;\;{1}/{30}+\;\;{1}/{90}=\;\;{3+1}/{90}

v=22.5cm

Which is positive so that it is 22.5 cm from lens B.

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