Book of Optics
One of his contemporaries, Kamal al-Din al-Farisi, offered the same experimentally-established explanation of the rainbow (without any contacts between them) in his "Kitab tanqih al-manazir" ("The Revision of the Optics"). Both authors however relied on the "Book of Optics" by Ibn al-Haytham (Alhacen)/ Alhazen.