प्रश्नानुशार, `(P_(1))/(P_(2))=(3)/(-2)`
`therefore (f_(1))/(f_(2)) = -(2)/(3)` अथवा `f_(2) = (-3)/(2)f_(1)`
यदि तुल्य फोकस-दूरी `f` हो, तो
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))=(1)/(f_(1))+(1)/(-(3)/(2)f_(1))=(1)/(f_(1))-(2)/(3f_(1))=(1)/(3f_(1))`
`therefore (1)/(30) = (1)/(3f_(1))` अथवा `f_(1) = 10` सेमी
तथा `f_(2) = -(3)/(2) xx 10 =-15` सेमी |