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A screen is placed 90 cm from an object....

A screen is placed 90 cm from an object. The image an object on the screen is formed by a convex lens two different locations separated by 20 cm. the focus length of the lense is

A

18cm

B

21.4cm

C

60cm

D

85.6cm

Text Solution

Verified by Experts

The correct Answer is:
B
From displacement method
`D=30cm` and d=20cm
`f=(d^(2)-d^(2))/40implies(90^(@)-20^(@))/(4xx90)`
`f=(110xx70)/(4xx90)impliesf=21.4cm`
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