Question

find the power of a concave lens of focal length 2m

Answer 1

p=1/f(m)
p=1/-2
the power is -0.5 dioptre

Answer 2

According to the sign convention , things present at the left of the lens are taken as negative..so we consider the  the focal length of concave lens as negative.
we will take f.length as -2. And for convex lens its taken as positive..
power=1/focal length
=1/-2
=-0.5 D