Question

A lens with f = +11cm is paired with a lens with f = −25cm. What is the focal length of the combination?

Answer 1

19.642 cm

Step-by-step explanation:

f₁ = Focal length of first lens = 11 cm

f₂ = Focal length of second lens = -25 cm

Combined focal length formula

`(1)/(f)=(1)/(f_1)+(1)/(f_2)\\\Rightarrow (1)/(f)=(1)/(11)+(1)/(-25)\\\Rightarrow (1)/(f)=(14)/(275)\\\Rightarrow f=(275)/(14)\\\Rightarrow f=19.642\ cm`

Combined focal length is 19.642 cm