mathematics, statistics, optics, engineering, radiometry, physics

A skew ray is a ray that has non-zero angular momentum about the symmetry axis of an optical system. In English, that means that the ray won't intersect the line down the middle of the lens. Rather than remaining embedded in a plane, a skew ray corkscrews through the system along a spiral; angular momentum is conserved, so a photon that starts out swinging around the center axis continues to swing around as it goes. Since light follows the path that takes the shortest time and most rays into an optical system are off axis in this way, the shortest path for most light through an optical system is, in fact, a helix.