Can you burn paper with moonlight? Part 2.

In this second video in this series, I present the proof of conservation of etendue that was left out in the first part:
• Can you burn paper wit...
Part 3 is available here
• Can you burn paper wit...
The treatment is highly inspired from the Wikipedia web page about etendue
en.wikipedia.o...

[@Woodpecker_18]

Love this video , do you have any recommendation for book to learn this type of physics or maths?

[@DeeperScience]

I don't know of any textbook, but you can start with the Wikipedia page mentioned in the description and go from there.