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The dispersive powers of glasses of lens...

The dispersive powers of glasses of lenses used in an achromatic pair are in the ratio 5 : 3. If the focal length of the concave lens is 15 cm , then the nature and focal length of the other lens would be

A

convex, 9 cm

B

concave, 9 cm

C

convex, 25 cm

D

concave, 25 cm

Text Solution

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The correct Answer is:
A
Condition for achromatic combination
`(omega)/(omega')=-(f)/(f')`
`(5)/(3)=(-(-15))/(f')`
`impliesf'=9cm`
Focal length is positive, so it is convex.
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