A lens is a homogeneous transparent medium, such as a glass bounded by two curved surfaces or one curved and one plane surface. This curved surface may be cylindrical, paraboloidal or spherical, but most are spherical. A simple lens is made up of just one transparent element, whereas a compound lens is made up of several simple lenses that are all aligned along the same axis. 

A lens can focus light rays to a specific point to generate a picture, unlike a prism, which refracts light before concentrating it. Lenses are used in a variety of image devices, including microscopes, telescopes, and cameras. Depending on their function, lenses can be convex or concave.

Important Terms Related to Lenses

Before we move further, it is important to know a few terms that are related to lenses:

• Focal Point:

The capital letter “F” is used to indicate the focal point of a lens. After going through a converging lens, the light beams converge at the focal point. However, the rays begin from a negative focal point before diverging through the lens in a diverging lens.

• Focal Length:

The focal length can be described as the distance between the focal point and the lens’s centre. 

• Principal Axis:

A hypothetical horizontal line drawn through the centre of the lens is the principal axis. The focal point of a perfect lens will be located on the principal axis at a range of the focal length from the lens’s centre.

• Refraction:

The light beams bend when a light wave travels from one medium to another. This is referred to as refraction. Lenses can bend several light rays through refracting medium. The majority of the lenses we use daily are made to bend rays of light to a precise focal point where objects will be in focus.

• Optical Centre:

If a ray of light is incident on a lens and after refraction (through the lens), the emergent ray is parallel to the incident ray, then the point at which the refracted ray intersects/appears to intersect the principal axis is called the optical centre of the lens.

Types of Lenses

Lenses can be of many types, some of them as stated below:

• Convex Lens – A convex lens is one in which the centre is thicker than the edges. 
• Concave Lens – When the centre of the lens is thinner than its rims, it is called a concave lens.
• Meniscus Lens- A meniscus lens has one concave side and the other side that is convex.
• Plano Lens- A flat lens is referred to as a Plano lens.

Advanced types of lenses

• Convexo-convex (bi-convex or double convex): Both surfaces are convex. The radii of the curvature of these surfaces may be equal or different.
• Plano-convex: One surface is plane, and the other is convex.
• Concavo-convex (convex meniscus): One surface is concave, and the other is convex. The radii of curvature of these surfaces are different. It is thicker in the middle.
• Concavo-concave (bi-concave or double concave): Both surfaces are concave. The radii of curvature of these surfaces may be equal or different.
• Plano-concave: One surface is plane, and the other is concave.
• Convexo-concave (concave meniscus): One surface is convex, and the other is concave. The radii of curvature of these surfaces are different. It is thinner in the middle. The line joining the centres of the curvature of the surfaces of the lens is called the principal axis of the lens.

Let’s learn about a few types of lenses in detail.

Converging Lens

A converging lens focuses parallel light rays to its main axis. Converging lenses are distinguished by their form, which is thick in the centre and thin at the top and bottom edges. 

Diverging Lens

A diverging lens diverges light beams that are going parallel to its main axis. Diverging lenses are further distinguished by their form, which is relatively thin in the middle and thick at the top and bottom.

Magnification of Lenses

Magnification is a term used to describe the process of enlarging something. It does not mean that the object becomes physically larger, but rather that it appears to be larger. Telescopic magnification and microscopic magnification are two kinds of magnification that can exist. 

When we place a small object in a big environment, microscopic magnification comes into effect. When larger items appear little, telescopic magnification enters the scene. A simple lens and a compound lens are the two types of magnification lenses now in use.

The factor by which an image grows in size as observed along the optical axis is known as longitudinal magnification.

Conclusion

Any sheet of refracting medium with at least a single curved surface is referred to as a lens. Lenses can be distributed into two types. Any transparent material that converges parallel beams of light down to a point is referred to as a converging lens. The middle of convergent lenses is thicker than the edges. A piece of reflective surface that causes parallel rays of light to seem to diverge from a point is known as a diverging lens. The margins of diverging lenses are thicker than the centre. The focal point is where rays of light that once parallel converged after coming from a converging lens or diverge after coming from a diverging lens.