Question

an object is kept at a distance of 4cm from the first focus of a convex lens. a real image is formed 9cm from the second focus. then the focal length of the lens is​

Answer 1

Answer:

+/- 6

Explanation:

u=−(4+f)

v=9+f

∴

⇒ 1/f= 1/9+f + 1/ 4+f

= 2f^2+13f = f^2 +13f +36

⇒f ^2 =36

⇒f=±6cm