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.

A joint Chinese and European X-ray telescope mission called Einstein Probe is successfully viewing the universe in widescreen, with a telescope design that mimics the eyes of lobsters. Einstein ...

A stunning new image from the James Webb Space Telescope (JWST) is allowing astronomers to examine the complex and turbulent ...

Once through the lens, the ray should pass through the principal focus. Draw a ray which passes from the object through the centre of the lens. Some ray diagrams may also show a third ray.