Focal length of concavelens

When used in light sparse medium, it can diverge the incident beam, so it is also called divergent lens. It is also called negative lens because its focal length is negative. For thin concave lens, the imaging formula, transverse magnification formula and sign rule are the same as those for convex lens.

The image formed by biconcave lens is always smaller than the vertical virtual image of the object. Biconcave lens is mainly used to correct myopia. Myopia is mainly due to the deformation of the lens, resulting in the premature collection of light in front of the retina. The biconcave lens plays the role of divergent light. The biconcave lens becomes an upright and reduced virtual image, which lengthens the image distance and just falls on the retina.

Lens is a kind of refractive optical element made of different optical materials. Its refractive surface is two spheres or one sphere. According to its shape, it can be divided into convex lens (positive lens) and concave lens (negative lens). The lens whose central part is thicker than the edge is called convex lens. According to their cross-sectional shape, there are biconvex lens, flat convex lens and concave convex lens. The lens whose central part is thinner than the edge is called concave lens. According to its different cross-sectional shape, it can be divided into double concave lens, flat concave lens and convex concave lens.