A lens is a curved piece of transparent glass that has the ability to concentrate or disperse light rays through refraction. Lenses are fundamental components in devices that manipulate light. Due to their properties, lenses are often found in telescopes, microscopes, and cameras, where they gather and focus light rays to create a clear image.
Cameras, for instance, don't rely on a single lens. Instead, they use a group of lenses to capture light effectively. The magnification of a lens refers to the relationship between the size of the image produced and the size of the actual object. To prevent image distortion or blurriness, multiple lenses can be used in conjunction.
Essential Terms Related to Lenses
Pole
The pole of a lens is the point where the principal axis intersects the lens's surface. This point is situated at the center of the lens's spherical refracting surface.
Optical Centre
The optical center of a lens is the point situated on the principal axis at the center of the lens.
Centre of Curvature
A lens has two spherical surfaces that form part of a sphere. The center of these spheres is known as the center of curvature.
Principal Axis
The principal axis is an imaginary line that passes through the centers of curvature and the pole of the lens.
Aperture
The aperture of a lens is the effective diameter of its light-transmitting area, or simply, the part of the lens that is suitable for refraction.
Focus
The focus is the point where collimated light parallel to the axis converges after passing through the lens.
Focal Length
The focal length is the distance between the optical center and the focus of the lens.
Power
The power of a lens is the reciprocal of its focal length, measured in units of diopters.
Lenses are classified into many types based on their structure and function. Broadly, they can be categorized as simple lenses or compound lenses.
A simple lens is a single piece of material that magnifies an object, while a compound lens is a combination of simple lenses arranged along a common axis. Compound lenses have the advantage of reducing optical aberrations, which are common in simple lenses. They also allow the magnification to be adjusted according to the user's needs.
Non-spherical Lenses
Aspheric Lens
An aspheric lens, also known as a non-spherical lens, is not part of a sphere or cylinder. Its complex surface reduces or eliminates optical aberrations compared to a simple lens. A single aspheric lens can replace multiple simple lenses, resulting in a more compact system.
Cylindrical Lens
Cylindrical lenses are unique in that they have curvature along only one axis. These lenses are used to convert elliptical light from laser diodes into a round beam or to focus light into a line. An example of such lenses is the anamorphic lenses used in motion pictures.
Fresnel Lens
A Fresnel lens is a lens with an optical surface divided into narrow rings, allowing the lens to be thinner and lighter than conventional lenses.
Other Types of Lenses
Lenticular lenses consist of a group of microlenses used in lenticular printing to create images with an illusion of depth. A bifocal lens has two or more graduated focal lengths. A gradient index lens has flat optical surfaces, while an axicon lens features a conical optical surface.
Concave and Convex Lenses
Lenses can also be classified based on their shape and function into two types:
Concave lens
Convex lens
A diagram illustrating a concave and convex lens is shown below:
Concave Lens
A concave lens is one where at least one side curves inwards. A biconcave lens is a concave lens with both sides curved inward. These lenses are diverging lenses, meaning they spread out light rays that pass through them. They are capable of diverging a parallel beam of light. For a concave lens, the edges are thicker than the center. They are used in eyeglasses to correct myopia or short-sightedness.
A concave lens produces an image that appears smaller to the viewer. The focal point of a concave lens is the point from which light rays parallel to the axis appear to diverge after passing through the lens. The distance from the optical center of the lens to the focal point is known as the focal length of the lens.
The image formed by a concave lens has the following characteristics:
It is located on the same side of the lens as the object
It is a virtual image
It is upright
It is smaller than the object
Regardless of the object's location, the image formed by a concave lens is always between the focal point and the optical center.
A concave lens always forms a virtual image. It can never form a real image. The image is always formed on the same side of the lens as the object, and can only be seen in the lens itself. It cannot be projected onto a screen. As the object's distance from the optical center increases, the size of the image decreases.
Convex Lens
A convex lens is a lens that curves outward. Unlike a concave lens, the center of a convex lens is thicker than its edges. Convex lenses are converging lenses, meaning they focus parallel light rays into a point. This point is known as the focal point of the convex lens, and the distance from the optical center to the focal point is the focal length. The focal point is on the opposite side of the lens from which the light rays originate.
A plano-convex lens is a convex lens with one flat side. The lens found in the human eye is a perfect example of a convex lens. Another common example is the magnifying glass used to correct hypermetropia or long-sightedness. Convex lenses are used in cameras where they focus light to produce clear images. They are also used in compound lenses, which are used in magnifying devices such as microscopes and telescopes.
When a linear object is placed perpendicular to the principal axis of the lens, a linear image is formed perpendicular to the principal axis due to the refraction of the lens. The position, size, and nature of the image formed depend on the position of the object relative to the lens.
The magnification of a lens is the ratio of the linear size of the image to the linear size of the object.
m = linear size of the image/linear size of the object
Optical Aberrations
Optical aberration is a defect that causes image distortion or blurriness. When this happens, the light is spread out instead of being focused on a specific point. Optical aberration is an undesirable property of lenses and can be eliminated by using a combination of lenses rather than a single lens.
There are several types of aberrations, including spherical aberration, chromatic aberration, and coma aberration, all of which can affect the quality and formation of an image.
Spherical Aberration
Spherical aberration is primarily caused by the fact that the spherical surfaces of a lens are not the ideal shape. As a result, beams that are parallel to but distant from the lens axis are refracted differently, causing image blur. Spherical aberration can be corrected using a lens with the correct surface curvature for a particular application.
Coma Aberration
Coma aberration occurs when an object is imaged off the optical axis of the lens. It can be eliminated by using a lens with curvatures that match the application. Best form lenses are often used for this purpose.
Chromatic Aberration
Chromatic aberration occurs due to dispersion. This happens when a lens fails to focus all colors to the same point. Chromatic aberration can be seen as colorful fringes around an image. It can be corrected using an achromatic doublet (or achromat).
Other types of aberrations include field curvature, astigmatism, and barrel and pincushion distortion.
Applications of Lenses
They are used in magnifying glasses.
Lenses are used in prosthetics to correct visual impairments.