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Two lenses in contact made of materials ...

Two lenses in contact made of materials with dispersive powers in the ratio`2:1`behaves as ab achromatic lens of focal length `10cm`.The individual focal length of the lenses are :

A

`5cm,-10cm`

B

`-5cm,10cm`

C

`10cm,-20cm`

D

`-20cm,10cm`

Text Solution

Verified by Experts

The correct Answer is:
A
Condition for Achromatism
`(omega_(1))/(f_(1))+(omega_(2))/(f_(2))=0`
`(omega_(1))/(omega_(2))=-(f_(1))/(f_(2))`
`2=-(f_(1))/(f_(2))`
`f_(2)=-(f_(1))/(f_(2))....(1)`
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2)) …..(2)`
From (1)&(2)
`f_(1)=-10cm`
`f_(2)=5cm`
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