What is plano convex lens and plano concave lens?

A lens is a transparent material (glass or plastic) with at least one curved surface. The word “lens” derives from the Latin word for "lentil" bean which has the shape of a Bi-convex lens.

The lens shape causes light rays to bend in a specific way as they pass through the lens itself, whether converging to a specific point or diverging as if from a specific point.

● Plano Concave Lenses - Plano Concave Lenses are negative focus length lenses that have a flat surface on one side and a spherical surface on the other. They are used to expand beams or to increase focal lengths in optical systems or other similar applications. Lenses can be anti-reflection coated.

● Plano Convex Lenses - Plano Convex Lenses are positive focus length lenses that have a flat surface on one side and spherical surface on the other. They are used for focusing beams in telescopes, collimator or condenser systems, optical transceivers or other applications. Lenses can be anti-reflection coated.

Index of refraction and Snell’s law - The difference between the index of refraction of the lens and the surrounding air causes the actual bending of the light rays. The bending phenomenon is described by Snell’s law for refraction. The law describes the relationship between the angle of incidence and refraction when passing a boundary between two different materials. It relates to the difference in the angle between the incident and refracted light rays to the indices of refraction for two materials.

Plano Convex Lenses are the best choice for focusing parallel rays of light to a single point, or a single line in the case of cylindrical lenses. This lens can be used to focus, collect and collimate light. It is the most economical choice for demanding applications. The asymmetry of these lenses minimizes spherical aberration in situations where the object and image are located at unequal distance from the lens. The optimum case is where the object is placed at infinity (parallel rays entering lens) and the final image is a focused point. Although infinite conjugate ratio (object distance/image distance) is optimum, plano-convex lenses will still minimize spherical aberration up to approximately 5:1 conjugate ratio. For the best performance, the curved surface should face the largest object distance or the infinite conjugate to reduce spherical aberration.

Plano Concave Lenses are the best choice where object and image are at absolute conjugate ratios greater than 5:1 and less than 1:5 to reduce spherical aberration, coma, and distortion. Plano-Concave lenses bend parallel input rays so they diverge from one another on the output side of the lens and hence have a negative focal length. The spherical aberration of the Plano-Concave lenses is negative and can be used to balance aberrations created by other lenses. Similar to the Plano-Convex lenses, the curvature surface should face the largest object distance or the infinite conjugate (except when used with high-energy lasers where this should be reversed to eliminate the possibility of a virtual focus) to minimize spherical aberration.