Question

Is the power of the concave lens positive or negative?

Answer 1

1. Power in optics is the capacity of the optical instrument to bend the light rays striking it. Power is defined as positive if an apparatus bends the light rays in the direction of the optical axis.
2. The reciprocal of a lens's focal length $\left(f\right)$ is the power of the lens $\left(P\right)$. The diopter, denoted by the letter $D$, is the SI unit of lens power.
   $p=\frac{1}{f}$
3. A concave lens will always have a negative focal length because it is a diverging lens.
4. The rays incident on a concave lens diverges. It is also referred to as a diverging lens because of this.

Hence, a concave lens is negative.