Assertion :Two convex lenses joined together cannot produce an achromatic combination. Reason: The condition for achromatism is $\frac{ω_{1}}{f_{1}} + \frac{ω_{2}}{f_{2}} = 0$ where symbols have their usual meaning.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Assertion is incorrect but Reason is correct

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Solution

The correct option is D Assertion is incorrect but Reason is correct
The condition given in the reason is correct. Hence

$\frac{f_{1}}{f_{2}} = - \frac{ω_{1}}{ω_{2}}$ which is negative.

Thus the lenses used for achromatic combination have to be of oppositely signed focal lengths.

Thus they cannot both be convex lenses.

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