A convex lens of focal length 30, cm and a co | vedclass

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(C) For the combination of lenses to produce parallel rays from parallel incident rays,the equivalent focal length of the system must be infinite $(F

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$20$

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(C) For the combination of lenses to produce parallel rays from parallel incident rays,the equivalent focal length of the system must be infinite $(F = \infty)$.The formula for the equivalent focal length of two lenses separated by a distance $d$ is given by:$\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} - \frac{d}{f_1 f_2}$Given:$f_1 = +30\, cm$ (convex lens)$f_2 = -10\, cm$ (concave lens)$F = \infty$Substituting the values:$\frac{1}{\infty} = \frac{1}{30} + \frac{1}{-10} - \frac{d}{(30)(-10)}$$0 = \frac{1}{30} - \frac{1}{10} + \frac{d}{300}$Multiplying by $300$:$0 = 10 - 30 + d$$0 = -20 + d$$d = 20\, cm$

$A$ convex lens of focal length $30\, cm$ and a concave lens of focal length $10\, cm$ are placed coaxially at a distance $d$ apart. Parallel rays are incident on the convex lens. The rays emerging from the concave lens are also parallel. Then $d = $ . . . . . . $cm$.

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