Power of Lenses

The Power of a Lens

1. The power of a lens is defined as the reciprocal of the focal length in unit meter. $$P=\frac{1}{f}$$ Important Note: f is in meter
2. The unit of power is diopter (D).
3. The relationship of the power with the thickness and types of lens are shown in the diagram below.

Lens	Power of the Lens
Converging (Convex)	Positive
Diverging (Concave)	Negative
Thick, with short focal length.	High
Thin, with long focal length.	Low

Thinner – Lower Power – Longer Focal Length

Thicker – Higher Power – Shorter Focal Length

Example:
The power of a lens is labeled as +5D. What is the focal length of the lens (in cm)? Is this a concave lens or a convex lens?
Answer:
$$\begin{array}{l}P=\frac{1}{f}\\ (+5)=\frac{1}{f}\\ f=\frac{1}{5}=0.2m\text{ = }20cm\end{array}$$
The power of the lens is positive. This is a convex lens.