How many foci does a lens have?

Every lens has two foci.

The focus is the point, or plane, at which light rays from infinity converge after passing through a lens and travelling a distance of one focal length. The distance from the lens to the focal point is called the focal length. The focal length is always positive for converging lenses, while diverging lenses always have negative focal lengths.

Types of lenses

A converging lens is a lens that converges rays of light that are travelling parallel to its principal axis.

  • Converging lenses can be identified by their shape; they are relatively thick across their middle and thin at their upper and lower edges.

A diverging lens is a lens that diverges rays of light travelling parallel to its principal axis.

  • Diverging lenses can also be identified by their shape; they are relatively thin across their middle and thick at their upper and lower edges.

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