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A lens is a piece of transparent material that has been finely shaped to refract light rays into an image. Lenses are a collection of small refracting prisms that refract light to create their own image. When these prisms work together, an image is focused at a single point.


Types of Lens

The form of lenses and the materials used to make them differ. They are divided into two groups based on the form and purpose of the lens: concave lens and convex lens. A concave lens has at least one inward curved side. A biconcave lens is a concave lens with both sides curled inward. The concave lenses are divergent, meaning that light rays refracted through them are stretched out. The convex lenses converge light rays parallel to their principal axis. Converging lenses are distinguished by their form; they are thick in the centre and thin at the top and bottom edges. A diverging lens diverges light beams parallel to its main axis.

Important Terms


The pole is the centre of the lens’s spherical refracting surface. The intersection of the principal axis with the lens surface is the pole.

Optical centre

The optical centre is the point on the principal axis at the lens’s centre.

Centre of curvature

A lens has two spherical surfaces that are joined together to produce a sphere. The centre of curvature is the point where these spheres intersect.

Principal axis

The principal axis is an imaginary line that passes across the pole and the centres of curvature.


Aperture refers to the portion of the lens that is suitable for refraction. The effective diameter of the lens’s light-transmitting region is called the aperture.


The point on which collimated light parallel to the axis is concentrated is called the focus.

Focal length

The focal length of a lens is the distance between its optical centre and its focal point, or focus.


The reciprocal of the focal length is the lens’s power. Dioptre is the SI unit of power.


Entropy is a measure of a system’s unpredictability or chaos. The amount of entropy in a system is determined by its mass. It is represented by the letter S, and its unit is joules per kelvin.

Entropy can be either positive or negative. The entropy of a system can only decrease if the entropy of another system increases, according to the second rule of thermodynamics.

As a block of ice melts, its entropy increases. It’s clear to see how the system’s disorder is increasing. Ice is made up of water molecules arranged in a crystal lattice. As ice melts, molecules gain energy, spread out more, and lose structure, resulting in a liquid. Similarly, changing from a liquid to a gas, such as water to steam, increases the system’s energy.

On the other hand, energy can decrease. This happens when steam turns into water or when water turns into ice. Because the matter is not in a closed system, the second law of thermodynamics is not violated. While the entropy of the system under investigation may decrease, the entropy of the environment increases.


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