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The focal length of field achromatic com...

The focal length of field achromatic combination of a telescope is`90cm`.The dispersive powers of lenses are`0.024`and`0.036`respectively .Their focal lengths will be-

A

`30cm and 60cm`

B

`45cm`and90cm

C

`15cm`and35cm

D

`30cm`and-45cm

Text Solution

Verified by Experts

The correct Answer is:
4
`because (omega_(1))/(f_(1))+(omega_(2))/(f_(2))=0`
`(1)/(f_(2))=-(0.024)/(0.036f_(1))`
`(1)/(f_(2))=-(2)/(3f_(1))`
`because(1)/(F)=(1)/(f_(1))+(10)/(f_(2))`
`because F=90cm`
`:.(1)/(90)=(1)/(f_(1))-(2)/(3f_(1))`
`f_(1)=30cm`
or`f_(2)=-(3xx30)/(2)=-45cm`
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