Question

A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point $15\,cm$ from the lens on the opposite side. If the lens is removed the point where the rays meet will move $5\,cm$ closer to the lens. The focal length of the lens is

• A. 5 cm
• B. -10 cm
• C. 20 cm
• D. -30 cm

Answer 1

Answer: (D)

-30 cm

Answer 2

$\frac{1}{ V }-\frac{1}{ u }=\frac{1}{ f }$
$\frac{1}{15}-\frac{1}{10}=\frac{1}{ f }$
$f =-30\, cm$