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# A system of lens consists of two convex lenses of focal length $f_1= 30\;cm$ and $f_2 =20\;cm$. Separated by a distance 'd'. When a parallel beam of light is incident on the first lens after two refraction the resultant rays come out parallel then the distance 'd' is

$(a)\;40\;cm \\ (b)\;50\;cm \\ (c)\;30\;cm \\ (d)\;60\;cm$

For lens 1, the parallel beam will coverage at focus P at $f_1=30\;cm$ from lens 1, and these rays if they are at focus of 2nd lens , they  will be rendered parallel.
$\therefore 'd'= f_1+f_2$
$\qquad= 30+20 =50 cm$
Hence b is the question.

edited Jul 15, 2014