A small bulb filament is placed exactly at the focal point of a convex lens. What emerges from the other side?

1. The paths of light rays are exactly reversible, so every ray diagram can be run backwards. 2. Rays arriving parallel to the axis cross at the focal point, so rays starting at the focal point must leave parallel to the axis. 3. A point light source at the focal point of a convex lens therefore sends out parallel rays on the other side. 4. This is how a directional beam is made from a source that emits in all directions, as in lighthouses and some traffic lights. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_