Optics is the branch of physics which deals with the behaviour of light waves.

A lens is a portion of a transparent material with two refracting surfaces such that at least one is curved with refractive index of its material being different from that of thhe surrounding.

A refractive medium surrounded by two spherical pages is called Lens. The lens is made of glass.

A thin spherical lens with refractive index greater than that of surrounding behaves as a convergent or convex lens.

If the portion of a lens is thinner than marginal one, it diverges parallel rays passing through it and behaves as divergent or concave lens.

The refractive index of glass is 1.5, the refractive index of water is 1.33 and the refractive index of carbon dioxide is 1.68.

Type of Lens

Lens are of two types:

Types of Lenses

Convex Lens

The lens that has both surfaces embossed in the middle is called a Convex lens. Convex Lens has a positive potential.

Concave Lens.

The lens that has both surfaces glued in the middle is called the Concave lens. The ability of concave lenses is negative.

Definition related to the Lens

Optical Center

The point inside the lens through which the light beam does not deviate after passing is called the optical center.

Principal Axis

The joining the center of curvature of both sides of the lens is called the principal axis of the lens OR C1 and C2 is a passing through optical center and perpendicular to the lens.

Principal Focus

A lens has two focal points. First focal point is an object point on the principal axis corresponding to which the image formed at infinity.

Focal Length

Focal length is defined as the distance between optical centre of a lens and the point where the parallel beam of light converges to appear to converge.


The rays coming parallel to the principal axis, the point at which it appears or appears after refraction of the lens, is called the focus of the lens.

Power of Lens

The inverse of a lens’s focus distance is called the power of the lens. The unit of power of Lens is the diopter that P represents. Lenses with 1-meter focus distance have a capacity of 1 diopter.

P = 1 / f (where P = diopter, f = focus distance)

Type of Images formed by Convex Lenses

The nature, size, position, . of an image made of convex lenses depends on the distance from the focus of the image.

Types of Images Formed by Concave Lenses

From concave lenses, the image is always formed between the lens and the focus from the object itself and the image is virtual, straight, and smaller than the object.

Joint Lens

Lanes formed by keeping two lanes adjacent to each other are called joint lanes. The capacitance of the combined lanes is equal to the sum of the capacities of the two lanes that P represents.

P = P1 + P2 (where P = combined lens capacity, P1 = first lens capacity, P2 = second lens capacity).

Method to Change in Lens Capacity and Focus Distance Change

Changes in lens capacity and focus distance are as follows:

  • If the lens is immersed in the same refractive index, the lens capacity becomes zero and the focus distance becomes infinite.
  • If the lens is immersed in a medium whose refractive index is higher than the refractive index of the lens, then the capacity of the lens decreases, and the focus distance increases, and the nature changes. For example, when a glass lens is immersed in water.
  • If the lens is immersed in a medium whose refractive index is less than the refractive index of the lens, then the capacity of the lens decreases, and the focus distance increases but in its nature, there is no change.

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