The Helmholtz reciprocity principle describes how a ray of light and its reverse ray encounter matched optical adventures, such as reflections, refractions, and absorptions in a passive medium, or at an interface. It does not apply to moving, non-linear, or magnetic media. For example, incoming and outgoing light can be considered as reversals of each other, without affecting the bidirectional reflect. PhysicsThe Stokes–Helmholtz reversion–reciprocity principle was stated in part by (1849) and with reference to. One simple yet important implication of this reciprocity principle is that any light directed through a lens in one direction (from object to image plane) is optically equal to its conjugate, i.e. light being directed through the same s.