# Physics Lesson 12.10.1 - Power of Lenses

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Welcome to our Physics lesson on Power of Lenses, this is the first lesson of our suite of physics lessons covering the topic of Power of Lenses. The Human Eye, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

## Power of Lenses

As we have seen in the previous tutorial, lenses are not all identical even when they belong to the same category. Their radius of curvature determines the point in which parallel rays coming from infinity converge. We called this point as focus, F and numerically, it was equal to half-distance from the lens to the centre of curvature (half of the radius).

There is a new quantity used to determine how able is a lens to collect rays at points near it. This quantity is the reciprocal of focal length and is known as (optical) Power of a lens, P. The only condition is that focal length must be written in metres. Its formula therefore is:

P = 1/F (in metres)

Power of lens is measured in Dioptre [D]. For example, a converging lens of focal length equal to 20 cm (0.2 m) has a power of

P = 1/F
= 1/0.2 m
= 5 dioptres

On the other hand, a diverging lens of focal length equal to 40 cm (0.4 m), has a power of

P = 1/F
= - 1/0.4 m
= - 2.5 dipotres

because the focal length in diverging lenses is taken as negative.

You have reached the end of Physics lesson 12.10.1 Power of Lenses. There are 3 lessons in this physics tutorial covering Power of Lenses. The Human Eye, you can access all the lessons from this tutorial below.

## More Power of Lenses. The Human Eye Lessons and Learning Resources

Optics Learning Material
Tutorial IDPhysics Tutorial TitleTutorialVideo
Tutorial
Revision
Notes
Revision
Questions
12.10Power of Lenses. The Human Eye
Lesson IDPhysics Lesson TitleLessonVideo
Lesson
12.10.1Power of Lenses
12.10.2Power of a Combination of Lenses
12.10.3The Human Eye

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