What is Optical Lens ?

Meniscus lenses — a third broad type also referred to as convex-concave — can be either positive or negative, depending on the curvature of both sides of the lens. A meniscus lens with a steep concave surface is negative, while a lens with a steeper convex surface will be positive. A meniscus lens with equal curvature on both sides would neither converge nor diverge light.

The conjugate ratio is defined as the ratio between the distance from the object (light source) to the lens and the distance from the lens to the projected image. The endpoints of these two lengths are known as the object and image points. These two points lie on the lens's optical axis and are positioned so that light emitted from the object point will be focused at the image point. An object placed at the focal point of a lens results in an infinite conjugate ratio, while an object placed at twice the focal length results in an image formed at twice the focal length, giving a conjugate ratio of 1.

Achromats (also known as achromatic lenses) are used to minimize a special type of image distortion called chromatic aberration. This distortion occurs when a lens fails to focus all color wavelengths to the same convergence point, resulting in blurred contrast and color fringing. Achromats use at least two separate lens elements —one high-dispersion concave and one low-dispersion convex — to achieve their corrective effect.

Fresnel lenses consist of thin, lightweight plastic sheets marked with a series of concentric grooves. Each groove serves as an individual refracting surface; the series of grooves bends collimated light to a common focal point. Fresnel lenses are a compromise between efficiency and optical quality: because the lens material is very thin, a very small amount of light is lost in the transmission process.

Gradient index (GRIN) lenses are simple planar lenses which continuously bend light rays within the lens until they finally converge on the focal point. This contrasts with conventional lenses, which primarily bend light abruptly when it exits the back of the lens material. GRIN lenses are therefore cost-effective and simple to employ. Additionally, the ability to precisely manufacture the length of the plane results in an enormous flexibility to fit application parameters.

After selecting the best lens type to fit their application, buyers should analyze the wavelength range of the application. When specifying a lens, manufacturers typically provide a range of electromagnetic radiation that the lens is designed to transmit. The wavelengths can be grouped into three broad groups: infrared, visible, and ultraviolet. A lens might not be limited to a single spectrum, and may be able to transmit wavelengths from both the infrared and visible range, or visible and ultraviolet range, etc.

Historically, optical lenses were constructed from transparent glass, but are now made from other materials — acrylics, polymers, and minerals — as well. Lens material is determined by the raw material's dispersion and wavelength characteristics. For example, a lens designed for applications demanding low dispersion might be made of crown glass. Acrylic and polymer lenses are best suited to transmission within the visible spectrum, while minerals such as germanium and sapphire are suitable for a very wide range of wavelengths but particularly excel within the infrared spectrum.

Single lenses are made up of two precisely regular opposite surfaces. These surfaces are curved; this curvature causes the incident beam to converge at a single point called the focal point. The distance at which the focal point is formed behind the lens gives the focal length of the image. The size of the image formed by the lens depends on the focal length of the lens.

Most lenses are spherical lenses and are made of two surfaces that are part of two spheres. Lenses are often cut or ground to impart a different shape. Lenses are broadly classified as single and compound lenses based on their construction. Detailed classification of lenses is given below:

Single lenses – further classified into biconvex, plano-convex, positive meniscus, negative meniscus, plano-concave and biconcave.
Compound lenses – array of single lenses
Cylindrical lenses – have curvature in one direction
Fresnel lenses – its optical surface is broken into narrow wings
Super lenses – made from meta materials with a negative refraction index
Lenticular lenses – arrays of micro lenses.

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction.

A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis

Lenses are made from materials such as glass or plastic, and are ground and polished or molded to a desired shape.

A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.

The word lens comes from lēns , the Latin name of the lentil, because a double-convex lens is lentil-shaped. The lentil plant also gives its name to a geometric figure.

Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia

The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass

Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses".

The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds (424 BC) mentioning a burning-glass

Pliny the Elder (1st century) confirms that burning-glasses were known in the Roman period.

Pliny also has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague)

Both Pliny and Seneca the Younger (3 BC–65 AD) described the magnifying effect of a glass globe filled with water.

Ptolemy (2nd century) wrote a book on Optics, which however survives only in the Latin translation of an incomplete and very poor Arabic translation.

The book was, however, received, by medieval scholars in the Islamic world, and commented upon by Ibn Sahl (10th century), who was in turn improved upon by Alhazen (Book of Optics, 11th century).

The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century (Eugenius of Palermo 1154).