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It is desired to make an achromatic comb...

It is desired to make an achromatic combination of two lenses, `(L_(1)& L_(2))` made of materials having dispersive power `omega_(1)` and `omega_(2)(ltomega_(1))`. If the combination of lenses is converging then:

A

`L_(1)` is converging

B

`L_(2)`is converging

C

Power of `L_(1)`is greater than the power of `L_(2)`

D

None of these

Text Solution

Verified by Experts

The correct Answer is:
B
Condition for achromatism
`(omega_(1))/(f_(1))+(omega_(2))/(f_(2))=0`
`(omega_(1))/(omega_(2))=-(f_(1))/(f_(2))`
Given that`omega_(1)gtomega_(2)`
`(omega_(1))/(omega_(2))gt1`
`-(f_(1))/(f_(2))gt1`
`f_(1)gtf_(2)`and of opposite nature
So,`P_(1) ltP_(2)`For focal length of combination
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))`
`P=P_(1)+P_(2)`
Since the combination of lens is converging that means `Pgt0`,,So power of`L_(2)` must be greater & must be positive .option (B)is correct choice.
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