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Focal point

From Encyclopædia

In OPTICS, the focal point of a lens is the point of convergence of incoming parallel light rays. If the lens is of the type that diverges light, its focal point is the apparent point of origin of the diverging rays. By symmetry, every lens has two focal points, one on each side of the lens and at the same distance from the center of the lens (the focal length). Because of the principle of the reversibility of light rays, the diverging light from a point source placed at the focal point of a converging lens will emerge as a parallel beam. This is known as collimating the light.

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