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InterviewSolution
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An achromaic convergent system of focal length `+20cm` is made of two lenses (in contact) of materials having dispersive powers in the ratio `1:2`.Their focal lenths must be respectivelyA. `10cm, -20cm`B. `20cm,10cm`C. `-10cm,-20cm`D. `20cm,-10cm` |
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Answer» Correct Answer - A `(omega_(1))/(f_(1)) +(omega_(2))/(f_(2)) = 0 rArr (omega)/(f_(1)) +(2omega)/(f_(2)) = 0` `f_(2) =- 2f_(1)` `(1)/(f_(1)) +(1)/(f_(2)) = (1)/(F) rArr (1)/(f_(1))-(1)/(2f_(1)) = (1)/(20)` `(1)/(2f_(1)) = (1)/(20) rArr f_(1) = 10cm, f_(2) =- 20cm` |
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