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(A) For a glass slab,the equivalent focal length $F$ is $\infty$.For two thin lenses of focal lengths $f_1$ and $f_2$ separated by a distance $d$,the

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$f_1+f_2$

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(A) For a glass slab,the equivalent focal length $F$ is $\infty$.For two thin lenses of focal lengths $f_1$ and $f_2$ separated by a distance $d$,the equivalent focal length $F$ is given by the formula:$\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} - \frac{d}{f_1 f_2}$Since the combination acts as a glass slab,we set $F = \infty$,which implies $\frac{1}{F} = 0$.Substituting this into the formula:$0 = \frac{1}{f_1} + \frac{1}{f_2} - \frac{d}{f_1 f_2}$$0 = \frac{f_2 + f_1}{f_1 f_2} - \frac{d}{f_1 f_2}$$\frac{d}{f_1 f_2} = \frac{f_1 + f_2}{f_1 f_2}$$d = f_1 + f_2$Thus,the distance of separation between the lenses must be $f_1 + f_2$.

For a combination of two convex lenses of focal lengths $f_1$ and $f_2$ to act as a glass slab,the distance of separation between them is

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