The Power of a Lens
- The power of a lens is defined as the reciprocal of the focal length in unit meter.
![](http://spmphysics.blog.onlinetuition.com.my/wp-content/uploads/sites/4/2020/02/formula-95-300x209.png)
(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 |
![](http://2.bp.blogspot.com/-xWbDu2fSsXg/UeGGn769-II/AAAAAAAACVY/EKPhp68iOrs/s400/Picture41.png) |
Thinner – Lower Power – Longer Focal Length |
![](http://3.bp.blogspot.com/-MYkYaEXVzsI/UeGGsmte6PI/AAAAAAAACVg/gmeEjBoLV9g/s400/Picture42.png) |
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:
![](http://spmphysics.blog.onlinetuition.com.my/wp-content/uploads/sites/4/2020/02/Light-formula1.svg)
The power of the lens is positive. This is a convex lens.