Ray Diagrams for Lenses

As with images formed by mirrors, it is convenient to locate the image of lenses by graphical methods. For the sake of simplicity, we consider the rays to bend at the plane through the center of the lens. For a converging lens, the principal rays are:

1.) The parallel ray, drawn parallel to the axis. This ray is bent through the second focal point of the lens.

2.) The central ray, drawn through the center (the vertex) of the lens. This ray is undeflected. (The faces of lens are parallel at this point, so the ray emerges in the same direction but displaced slightly. Since the lens is thin, the displacement is negligible.)

3.) The focal ray, drawn hrough the first focal point. This ray emerges parallel to the axis.

The principal rays for a diverging lens are:

1.) The parallel ray, drawn parallel to the axis. This ray diverges from the lens as if it came from the second focal point.

2.) The central ray, drawn through the center (the vertex) of the lens. This ray is undeflected.

3.) The focal ray, drawn toward the the first focal point. This ray emerges parallel to the axis.