Some ray diagrams may also show a third ray. Peep holes contain concave lenses. They are set into doors so the occupant can identify a visitor before opening the door.

Using data from 10 days of observations on the Chandra X-ray telescope and the European Space Agency’s XMM-Newton, Binder and ...

In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.