Ray tracing involves tracking certain *easy rays* through the lens. With
convex lenses, these are

- A horizontal ray is refracted through the far focal point.
- A ray that strikes the middle of the lens continues in a straight line without bending.
- A ray that passes through the focal point and strikes the lens, or a ray that seems to have come from the focal point, will leave the lens horizontal.

**Lesson 1**, convex lens with an object far from
the lens.

**Lesson 2**, convex lens with an object near the
lens.

The rules for concave lenses, are similar:

- A horizontal ray is refracted outward, as if it were emanating from the near focal point.
- A ray that strikes the middle of the lens continues in a straight line without bending.
- A ray coming from the object on course for the far focal point will leave the lens horizontal.

**Lesson 3**, concave lens. (Note that object placement
has little effect on the nature of the image with concave lenses.)

In every case, convex or concave, if the rays leaving the lens actually intersect,
the image is **real**. If the rays leaving the lens must be extrapolated
back through the lens to find a point of intersection, the image is** virtual**.