Question

Assertion :Two convex lenses of same material and equal focal length ${}^{\prime }f{}^{\prime }$ from achromatic combination when separated by distance ${}^{\prime }f{}^{\prime }$. Reason: $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1{f}_2}$ where $F=$ equivalent focal length of lenses having focal lengths $f_1$ and $f_2$ separated by a distance ${}^{\prime }d{}^{\prime }$

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

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

For achromatic combination of two convex lenses of same material separated by distance $x$ such that,

$\frac{\omega }{f_1}+\frac{\omega }{f_2}=\frac{2x\omega }{f_1{f}_2}$

Hence the Reason is not the correct explanation for Assertion.