Question

why is power positive for convex lens and negative for concave lens??

Answer 1

The focal length for the convex lens is positive and the focal length of the concave lens is negative in nature.Also, the power of the lens is defined as the strength of the lens in terms of power. That is mathematically we can write as,
$Power=\frac{1}{focallength}$
As, the focal length of the convex lens is positive therefore power is positive and as the focal length of the concave lens is negative therefore it has a negative power.