Power of a Lens

One of the most intriguing ideas in ray optics is the power of a lens. In short, the ability of a lens to bend light is its strength in ray optics. A lens’s ability to refract light passing through it is directly proportional to its power. The converging ability of a convex lens is its power, and diverging ability of a concave lens is its diverging ability.

Refraction

When light travels obliquely from one medium to another medium, the direction of light propagation changes in the second medium, this phenomenon is called refraction.

Lens

A lens is a piece of glass which focuses light rays to a point or diverges light from a concentrated point. It converges or diverges with the help of dispersion. A simple lens is made of a transparent material, while the compound lens is made of multiple lenses.

Types of Lens

There are two types of lenses which are given below

1. Convex Lens
2. Concave Lens

Convex Lens

A convex lens has spherical surfaces that protrude in the centre. Convex lens is thicker in the middle part and thinner at the edges of the lens. This lens converges or focuses parallel rays of light which fall on its surface. Thus, a convex lens is also called a converging lens. A convex lens produces real and virtual images depending on the distance of the object from the lens.

Concave Lens

A concave lens has a spherical surface which curves inward. A concave lens is thinner in the centre and is thicker at the edges. A concave lens diverges or scatters parallel rays of light which falls on its surface. Thus, concave lens is also considered as a diverging lens. This lens always forms a virtual, upright (erect) image of a body, regardless of its distance from the lens.

Focus or Focal Point

Focal point is the point where the light coming from infinity converges after it passes through the lens. It is symbolically represented as F.

Focal Length

Distance between the pole and focal point of a lens is called focal length. It is represented as f.

There are two types of focal length which are based on the focal point.

Negative Focal Length

A negative focal length is the focal point which is on the same side of the lens as the body. Generally negative focal length is obtained in a concave lens, when the parallel light ray which travels parallel to the principal axis seems to diverge from the second focus, the focus is termed as the virtual focus and its focal length is assumed to be negative.

Positive Focal Length

A positive focal length is the focal point of the lens which is on the opposite side of the lens from the position of the object. It is generally observed in a convex lens when the parallel beam or ray of light which travels parallel to the principal axis meets actually at a point, the focus is termed as the real focus and its focal length is assumed to be positive.

Power of a Lens

In Ray Optics, the power of a lens is the ability to bend light. The greater the power of a lens, the greater the ability of the lens to refract light which passes through it. The power defines the converging power of a converging lens and the diverging power (ability) of a concave lens. The number of light curves (bends) increases with decreasing focal length. Therefore, the power of a lens is inversely proportional to the focal length of that lens. A short focal length essentially causes high optical power.

Power of a lens formula

Power of a lens formula is given as

Power = 1/(focal length)

Therefore, power of a lens is