Question

Find the focal length of a lens of power −2.0 D. What type of lens is this?

Answer 1

Power of lens, 

P = −2.0 D

P = `1/"f"`

∴ f = `1/"P"`

= `1/(-2.0  "D")`

= −0.5 m

A concave lens has a negative focal length. Hence, it is a concave lens.