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principal focus| Point towards which incident rays parallel to the principal axis of a lens converge, or from which they diverge, after refraction. The distance from the lens to its principal focus is its focal length. |
| The principal focus of a converging (convex) lens or of a parabolic concave mirror is the point at which parallel incident rays will converge when refracted or reflected. It is a real focus. |
| The principal focus of a diverging (concave) lens is the point from which incident rays parallel to the principal axis appear to diverge after refraction. It is a virtual or imaginary focus at which no light rays actually meet. |
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