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Why can't you collimate incoherent light as well as a laser?

Ciro Santilli (@cirosantilli, 37) ... Science Natural science Physics Condensed matter physics Laser Lasers vs other light sources
Updated 2025-07-16  0 By others on same topic  0 Discussions Create my own version
physics.stackexchange.com/questions/252393/why-cant-incoherent-light-be-collimated-as-well-as-laser-light-e-g-in-a-laser
You could put an LED in a cavity with a thin long hole but then, most rays, which are not aligned with the hole, will just bounce inside forever producing heat.
So you would have a very hot device, and very little efficiency on the light output. This heat might also behave like a black-body radiation source, so you would not have a single frequency.
The beauty of lasers is the laser cavity (two parallel mirrors around the medium) selects parallel motion preferentially, see e.g.: youtu.be/_JOchLyNO_w?t=832 from Video "How Lasers Work by Scientized (2017)"

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