Understanding the Power of Lens

The power of a lens determines the divergence or convergence of light produced by the lenses. This power of the focal point lens is the digression by which it meets or separates a light emission falling at a unit far off from the optical focus. The power of a lens can be positive or negative depending on the type of lens, i.e. it is positive for a converging lens and negative for a diverging lens. As the focal length increases, the number of light bends decreases. Hence, the focal length of the lens is inversely proportional to the intensity. 

Defining the focus and focal length of a lens

A focus or a focal point is the point responsible for converging light coming from infinity after passing through the lens. It is denoted by F. The distance to this focal point from the pole of the lens is the focal length of a lens. This is marked as f. The focal length can be of two types, i.e. positive focal length or negative focal length.

1. Positive focal length

In this case, the focus area is on the other side of the lens where the object is. Normally, it is seen with a convex lens when the beam of light moves parallel along the main axis and meets somewhere. Here the focus is real and can be called a real focus.

2. Negative focal length

In this case, the focus area is on the same side as the object. Typically, it is obtained with a concave lens when the beam of light moves parallel with the main axis and appears to vary from location or secondary focus. Here the focus is virtual and can be called a virtual focus.

Thus, we can say that when the lens converges light, the focal length is said to be positive and negative when the lens diverges light. Also, with a shorter focal length, the rays can be made to bend more sharply, which helps to diverge them more and brings them to a focus at a nearby distance.

The Power of lens 

In ray optics, the power of the lens gives the ability to bend light. Hence, when the lens has more strength, more is its ability to refract light passing through it. Power is what defines the converging and diverging ability of a convex and a concave lens. The number of light bends tends to decrease with the increase in focal length. As a result, we can say that the power of the lens is inversely proportional to its focal length. Thus, a short focal length leads to high power in optics. The power is 0 in the case of a plane glass plate. 

The power of a lens can be written as:

Power (P)=1Focal Length (f)

Refraction and Refractive Index

A lens is a transparent piece of glass used to focus or distribute a beam of light with the help of the refraction property of light. The change seen in the direction of the path when light travels from one medium to another is called refraction. The refractive index determines the ability of objects to refract light. It denotes the total amount of refraction that can exist in an object. A refractive index has no unit. The refractive index in vacuum is 1. One can calculate the refractive index using the formula stated below:

n=cv

Here, n = refractive index of the medium, c = speed of light in vacuum and v = speed of light in a medium.

The power of lens using refractive index

The power of a lens is said to have a relation with the refractive index using a formula popularly known as the Lens maker’s formula. The formula works by having the radius of curvatures of both surfaces. The lens is part of an empty glass sphere. The radius of curvature denotes the radius of the sphere. Each lens contains two radical curvatures, say R1 and R2. 

The formula for the lens maker’s formula is:

1f=(n-1)*(1R1–1R2)

Here, n = refractive index of the medium, f = focal length of a lens, R1 = Radius of curvature for surface 1 and  R2 = Radius of curvature for surface 2. This formula can be used to find the power of the lens using the refractive index of the material and radius of curvatures of the lens.

Conclusion

Lenses are used to focus light. The most common lenses we see are human mirrors. One small lens is also present in each of our eyes. Now, if this lens is not properly aligned, it decreases the light being received and it will be difficult to see. Hence, the need to put another lens in front of the eyes to help focus the light better and see clearly. Lenses are used in various imaging devices such as telescopes, cameras, etc. They are used as visual aids to treat the two most common defects of vision – myopia and hypermetropia. The focal point shines the light to a point and structures the picture. It’s mainly used as a contact focal point in our spectacles, in the amplifying glass, magnifying instruments and so on.