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.
Power of the Lens
|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.