Title page of a 1572 edition of 'Opticae Thesaurus' or 'Book of Optics' by the Arab mathematician, astronomer and physicist Hasan Ibn al-Haytham, known as Alhazen (965-1040). In this seven volume book Alhazen described his experiments with the first camera obscura, which demonstrated that light travels in straight lines, and claimed that light can be either primary, emitted by self-luminous bodies, or secondary, which is primary light reflected by another body. He also proposed that vision is the result of light entering the eye, rather than the eyes emitting radiation onto objects.