# Light box and oblique parallel rays – concave lens

In the previous section, we showed a simple method to find the principal focal point of a convex lens.

Hopefully as a keen scientist, you have experimented further with the light box and projected the three parallel light streams at an oblique angle to the concave lens.

This is very unlikely to come up in an exam, but what is interesting to note is that even at this oblique angle, you can still measure the principal focal length as shown below:

The principal focal plane contains the principal focal point and is perpendicular (at right angles) to the principal axis.