The intellectual lights of his Court were 'Ali ibn Ydnus (d.

The intellectual lights of his Court were 'Ali ibn Ydnus (d. 400/ 1009), the greatest astronomer Egypt has ever produced, and abu 'Ali al­Haitham (Latin Alhazen), the principal Muslim physicist and student of optics. The latter was undoubtedly the foremost physicist of the Middle Ages. His researches into geometrical and physiological optics were considered to be the most important and useful up to the time of Renaissance.

His expla­nation of the vision and functions of the eye was far in advance of the ideas of the ancients. The chief work for which he is noted is one on optics, Kitab al-Manazir, of which the original is lost but which was translated into Latin in the sixth/twelfth century. Almost all the medieval writers on optics in the West based their works on ibn Haitham's Opticae Thesaurus.

In this work he opposed the theory of Euclid and Ptolemy that the eye sends out visual rays to the object of vision, and presented experiments for testing the angles of incidence and reflection. In certain experiments he approached the theoretical discovery of magnifying lenses which were manu­factured in Italy centuries later.[^4] Ibn al-Haitham was the greatest Muslim physicist and one of the foremost opticians of the world.

He found out the law of refraction in transparent bodies; laws of reflection of light; spherical and parabolic aberrations; and the law of refraction which later came to be known as Snell's Law. He discussed the magnifying power of a lens, refraction of light in the earth's atmosphere, and beginning or termination of twilight when the sun is 19° vertically below the horizon. He tried by these means to estimate the height of the homo­geneous atmosphere.

He gave a better explanation of vision, though he erroneously assumed the lens of the eye to be the organ of sight. Later on ibn Rushd corrected this error and showed that sight is the function of the retina. Ibn al-Haitham explained the vision of a body by the aid of two eyes and the more magnified appearance of heavenly bodies when near the horizon than when vertically higher. Muslim scientists evinced much interest in the determination of specific gravity of bodies.

At Ghaznah in eastern Afghanistan lived abu al-Raiban Muhammad ibn Ahmad al-Biruni (363-440/973-1048), considered one of the most original and profound scientists that the medieval world produced in the domains of physical and mathematical sciences.