The Focal Length of Convex Lens is 20 cm, What is its Power?

Discover the power of a convex lens with a 20 cm focal length and learn how it impacts optical devices and everyday life. Explore the calculation of power and its real-world applications.

The Basics of Convex Lenses

Convex lenses are one of the most commonly used optical devices in the world. These lenses are thicker at the center and thinner at the edges, converging light rays to a focal point. One of the key characteristics of convex lenses is their focal length, which plays a crucial role in determining the lens’s power.

Understanding Focal Length

The focal length of a lens is the distance between the lens and its focal point. In the case of a convex lens, if the focal length is 20 cm, it means that parallel rays of light entering the lens will converge to a point 20 cm away from the lens.

Calculating Power

The power of a lens is defined as the reciprocal of its focal length in meters. So, for a convex lens with a focal length of 20 cm (0.2 m), the power can be calculated as:

Power = 1 / Focal Length

Power = 1 / 0.2

Power = 5 diopters

Importance of Power in Lenses

The power of a lens determines its ability to converge or diverge light rays. A lens with a higher power will have a shorter focal length and will converge light more strongly than a lens with a lower power.

Real-World Applications

Convex lenses with a 20 cm focal length and 5 diopters of power are commonly used in eyeglasses to correct nearsightedness. These lenses help to converge light rays in a way that brings distant objects into focus for the wearer.


In conclusion, the focal length of a convex lens is a crucial parameter that directly impacts the lens’s power. By understanding this relationship, we can appreciate the importance of focal length in optical devices and its practical applications in everyday life.

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