Question

The power of a lens is, −2 D. What is its focal length? 

Answer 1

​Power of a lens is given by: 

 power p=1focal length">power p=`1/focal length`  

`p=1/f` 

`⇒f=1/p⇒f=1/-2f=-0.5 m` 

Negative sign indicates that it's a concave lens.