Lens systems
Lens systems make use of the laws of refraction and reflection. They affect the direction in which light travels and create an optical image. Lenses differ according to their optical effect and geometrical design:
Converging or convex lenses A convex lens reduces the divergence of a light beam. One or both of its surfaces is spherical so that it is thicker in the middle than at the edges. Light passing through a convex lens parallel to the optical axis converges at the focal point ¦. The distance from the focal point to the lens is called the focal distance.
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Divergent or concave lenses A concave lens increases the divergence of a light beam. One or both of its surfaces is spherical so that it is thinner in the middle than at the edges. Light passing through a concave lens parallel to the optical axis diverges as if it came from a focal point in front of the lens. The distance between the lens and this focal point is the focal length. In this case, it has a negative value and is also known as the diffusion length. The reciprocal of the focal length is called the refractive index D.
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