> I sometimes feel like many systems built are just much to over-engineered and don't even attempt to exploit physics to do their job. PAPI lights are my favou

> I sometimes feel like many systems built are just much to over-engineered and don't even attempt to exploit physics to do their job. PAPI lights are my favourite example of this. For those that don't know the system, PAPI lights are installed next to runway touchdown point and give visual feedback of vertical approach path: are you too low, too high or just fine. Photo: https://i.imgur.com/Da8p3Uk.jpg https://i.imgur.com/Da8p3Uk.jpg Diagram: https://i.imgur.com/XSrUeD1.jpg https://i.imgur.com/XSrUeD1.jpg It needs no moving parts and no electronic control. Each light has a filter that splits the beam into white and red sectors. Red is shown below a certain angle, white above. Each light is at a different elevation angle. Photo: https://i.imgur.com/D0GvPHA.png https://i.imgur.com/D0GvPHA.png Diagram: https://i.imgur.com/o2SRGE8.jpg https://i.imgur.com/o2SRGE8.jpg (red lines are red/white sector boundary for each light) Viewing from a very low angle, all four lights look red. Slightly above that, one of them turns white. At standard approach angle, two are white and two are red. A bit above approach path, the third one becomes white. At very high angles, the whole row is white. PAPI lights become more precise as the aicraft gets closer. The system can serve any number of aircraft at the same time, does not have to track them, and does not require receivers/transmitters or any other on-board equipment.