The double concave lens is similar to the flat concave lens. The focal length is negative, but the light incident in parallel diverges outward. The curvature radii of the two sides of the double concave lens are equal. It is generally used for beam expansion and projection.

## Introduction to biconcave lens

The double concave lens is similar to the flat concave lens. The focal length is negative, but the light incident in parallel diverges outward. The curvature radii of the two sides of the double concave lens are equal. It is generally used for beam expansion and projection.

## Focal length of biconcave lens

As shown in Fig. 1, the spherical radius corresponding to the front concave surface of the biconcave lens is R1, and the spherical radius corresponding to the rear concave surface of the biconcave lens is R2.

## Principle of biconcave lens

The principle of geometric drawing of biconcave lens imaging is the same as that of convex lens. From the top of the object, it is also regarded as two straight lines: one is parallel to the main optical axis, deflects into divergent light after passing through the biconcave lens, and returns the refracted light to the main focus in the opposite direction; The other passes through the optical center point of the lens. The two straight lines intersect at one point, which is the image of the object.

## Difference between double concave lens and flat concave lens

Convex concave lens for myopia; Concave convex lens is used for reading glasses; Plano convex lens for instrument or amplification; Flat concave lenses are used in instruments.

A flat concave lens is a lens with one side concave and one side flat, while a biconcave lens is a lens with both sides concave.

## Application of biconcave lens

The line connecting the centers of curvature on both sides is called the principal axis, and the central point O is called the optical center. Light passing through the optical center is not refracted no matter where it comes from. The light beam parallel to the main axis is refracted on the concave lens and diverges to the four directions. The extension line opposite to its divergence direction will be at a point F on the same side as the light source, and its refracted light will just be emitted from point F, which is called the virtual focus. One on each side of the lens. Biconcave lens is also called divergent lens. The focal length of a concave lens refers to the distance from the focal point to the center of the lens. The larger the radius of the lens, the longer the focal length. If it is a thin lens, the focal lengths on both sides are equal.

When used in light sparse medium, it can diverge the incident beam, so it is also called divergent lens. Also known as 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 of 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 forms an upright and reduced virtual image, which lengthens the image distance and just falls on the retina.

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