Use the following ray diagram, Fig. 6(b).56 to calculate focal length of lens `L_(2)`.
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For lens `L_(1), f = 15 cm`, `u = -40cm`, `v = ?` `(1)/(v) = (!)/(f) + (1)/(u) = (1)/(15) - (1)/(40) = (5)/(120), v = 24 cm` The image would have been at `I'`, where `L_(1)I' = 24 cm`. `I'` serves as object for `L_(2)` `u = + (24 - 14) = 10 cm, v = 30 cm, f = ?` `(1)/(f') = (1)/(v) - (1)/(u) = (1)/(30) - (1)/(10) = (1)/(15) f = - 15 cm`
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