"Far from being mere intermediaries, the Arabs entirely recast the science of optics. It is difficult to explain the occurrence of such an important change in a society which had renounced representational images. There is a hypothesis, difficult to verify, that it was this renunciation which allowed the abandonment of the perspective cone" [of Greek optics] "in favour of a ray of light which started from the object and entered into the eye. On the basis of this radically new idea, Arab physicists not only improved the quality of optical tables, but, most important, they restored optics to its previous unity. The most important figure is al-Haytham, known to the West as Alhazen." (Authier, 1995)
The apparent difficulty disappears when al-Haytham's text is compared with Varahamihira's position on light, which shows the origin of al-Haytham's "radically new idea":
Varahamihira stated that reflection is caused by the back-scattering of particles and refraction (the change of direction of a light ray as it moves from one medium into another) by the ability of the particles to penetrate inner spaces of the material, much like fluids that move through porous objects. (South Asian History Project, 2002)
In his Discourse on Light Al-Haytham compares light rays with "arrow balls" and writes: "Lights which propagate themselves in transparent bodies propagate themselves with a very rapid motion, imperceptible on account of its rapidity. However, their motion in thin substances, that is to say those that are diaphanous, is more rapid than their motion in thick substances... In fact, when light travels through it, every diaphanous substance opposes to it a slight resistance which depends on its structure." (quoted in Authier, 1995)
Al-Biruni, a contemporary of Al-Haytham, had travelled to India and translated some of Varahamihira's work. Given the active exchange of ideas between the various Arab centres of learning it appears most unlikely that al-Haytham did not know of Varahamihira's theory of light.
Authier, M. (1995) Refraction and Cartesian 'Forgetfulness'. In: M. Serres (editor): A History of Scientific Thought, Elements of a History of Science. Blackwell, Oxford, 315 - 343. (Translation of Éléments d'Histoire des Sciences, Bordas, Paris, 1989)
South Asian History Project (2002) History of the Physical Sciences in India. http://india resource.tripod.com/physics.htm, last update 11 February 2002.