This video presents the physics behind the impossibility of concentrating light beyond a certain point. More information can be found at:

https://whatif.xkcd.com/145/
https://en.wikipedia.org/wiki/Etendue
https://www.askamathematician.com/201...
https://physics.stackexchange.com/que...
https://physics.stackexchange.com/que...

Note that many comments in the last three links have errors, which are discussed in the video.

The Cooke paper is
Dave Cooke" Sunpumped lasers: revisiting an old problem with nonimaging optics", 20 December 1992 / Vol. 31, No. 36 / APPLIED OPTICS 75417546

Part 2 and 3 of this series:
   • Can you burn paper with moonlight? Pa...  
   • Can you burn paper with moonlight? Pa...