Question

A concave lens is placed in contact with a convex lens of focal length 25 cm. The combination produces a real image at a distance of 80 cm, If an object is at a distance of 40 cm, the focal length of concave lens is

• A. -400 cm
• B. -200 cm
• C. +400 cm
• D. +200 cm

Answer 1

Answer: (A)

-400 cm

For the combinationof lens,
Here, u = -40 cm , v = +80cm , f = ?

$\therefore$

$\frac{1}{f}$ = $\frac{1}{80}-\frac{1}{-40}$

$f=\frac{80}{3}$

Now, $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}\Rightarrow \frac{1}{f_2}=\frac{1}{f}-\frac{1}{f_1}$

$\frac{1}{f_2}=\frac{3}{80}-\frac{1}{25}$ =
$\frac{15-16}{400}=\frac{-1}{400}\Rightarrow f_2=-400cm$