The Power of a Lens
- The power of a lens is defined as the reciprocal of the focal length in unit meter.
(Important Note: f is in meter} - The unit of power is diopter (D).
- 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:
The power of the lens is positive. This is a convex lens.
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:
The power of the lens is positive. This is a convex lens.