Mammoth Memory

Light box and oblique parallel rays

This is very unlikely to come up in an exam, but it's still useful to know.

If you project the three parallel light streams at an oblique angle to the convex lens, you can still measure the principal focal length – see below:

Ray diagram of a light box at an oblique angle to a convex lens

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

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