Ibn al-Haytham: 2 ideas and puzzles worth knowing

2 ideas and puzzles from Ibn al-Haytham, each explained in a minute and credited to the original work. Free on Savvy.

On a circular billiard table, hit the ball so it bounces off the rail into a second ball
Two balls sit anywhere on a circular billiard table. Using only the cushion — no other balls, no pockets — where on the rail must you aim so that a ball struck from the first point…
— Ibn al-Haytham (Alhazen), Book of Optics (Kitab al-Manazir)
An 11th-century book insisted you should test your theories, not just argue for them
Ibn al-Haytham (Alhazen), under house arrest in Cairo, wrote the Book of Optics, correctly explaining that vision works by light entering the eye rather than the eye emitting rays.…
— Ibn al-Haytham, Book of Optics (Kitab al-Manazir)

Five ideas worth knowing, every week