Abstract:  The equations of emergent light produced by a point object located on the axis of a len are derived without considering paraxial approximation by using a ray-tracing method. Based on the equations, the distribution diagrams of emergent light field are drawn by using Geometric Sketchpad. The results obtained by analyzing the diagrams show that when the len is convex, the energy of the actual emergent light field in the initial range behind the len is mainly distributed in the zone formed by a rotation surface with a special generatrix and a spin axis which is the optical axis. The border of the zone becomes more and more blurred as the distance from the lens increases. On each cross section perpendicular to the optical axis, the actual emergent light field presents three different characteristics successively with the distance from the len increasing. When the len is concave, there is not a zone where the light intensity is greatly stronger than another zones in the emergent light field, and the energy scatters in every cross section.

Key words:  convex lens, concave lens, paraxial approximation, ray-tracing methods, Geometric Sketchpad