Lens

Lenses are basically magnifying glasses with curved sides. A lens is a piece of transparent glass which concentrates or disperses light rays when passes through them by refraction. Due to the magnifying property, lenses are used in telescopes and other magnifying devices. They are employed in cameras for gathering the light rays.

In cameras, not one, but a group of lenses are used for the gathering of light. Magnification of a lens is the relation between the size of the image formed and the size of the object. Lenses can also be used in groups in order to avoid the blurriness or distortion caused to the image formed by the lens.

Related Terms

Pole

The centre of the spherical refracting surface of the lens is called the pole. The point where the principal axis meets the surface of the lens.

Optical Centre

The point on the principal axis at the centre of the lens is called Optical centre.

Centre of Curvature

A lens has two spherical surfaces, these two spherical surfaces form a part of a sphere. The centre of these spheres is known as the centre of curvature.

Principal axis

The principal axis is an imaginary line passing through the centres of curvature and the pole.

Aperture

The area of the lens suitable for refraction is called Aperture. The aperture of the lens is the effective diameter of its light-transmitting area.

Focus

Focus is the point onto which collimated light parallel to the axis is focused.

Focal length

The focal length is the distance between the optical centre and the focal point or focus of the lens

Power 

The power of the lens is the reciprocal of its focal length. The S.I unit of power is Dioptre.

 

Types of Lenses

Today, we have a lot of different lenses. Generally, they are either categorized as a simple lens or a compound lens.

A simple lens is a single piece of magnifying material while a compound lens consists of a number of simple lenses arranged along a common axis. Optical aberrations are sometimes found in simple lenses while this property is eliminated in a compound lens. Another advantage of the compound lens is that the magnification of the lens can be adjusted as per the user’s requirements.

Non-spherical Lenses

Aspheric Lens

Aspheric lens is often known as a non-spherical lens. Aspheric lens is a lens whose surface is not part of a sphere or a cylinder. The complex surface of an aspheric lens reduces or eliminates optical aberrations as compared to a simple lens. A single aspheric lens can replace a combination of simple lenses resulting in a system with a much-reduced size.

Cylindrical Lens

Lenses that have a curvature along only one axis are classified as cylindrical lenses. Their main purpose is to convert laser diode elliptical light into a round beam or to focus light into a line. Motion picture anamorphic lenses are an example of such lenses.

Fresnel Lens

A Fresnel lens is a lens whose optical surface is divided into narrow rings. This allows the lens to be much thinner and lighter than conventional lenses.

Other Lenses

Lenses that have a curvature along only one axis are classified as cylindrical lenses. Their main purpose is to convert laser diode elliptical light into a round beam or to focus light into a line. Motion picture anamorphic lenses are an example of such lenses.

Lenticular lenses are a group of microlenses that are used in lenticular printing. These lenses produce images that have an illusion of depth.

A bifocal lens has two or more or graduated focal lengths.

A gradient index lens is a lens with flat optical surfaces while an axicon lens features a conical optical surface.