Abū Alī al-Ḥasan ibn al-Ḥasan ibn al-Haytham

Abū Alī al-Ḥasan ibn al-Ḥasan ibn al-Haytham, Persian, Alhacen or Alhazen (965 in Basra - c. 1039 in Cairo), was an Arabor Persian polymath. He made significant contributions to the principles of optics, as well as to anatomy, astronomy, engineering, mathematics, medicine, ophthalmology, philosophy, physics, psychology, visual perception, and to science in general with his introduction of the scientific method. He is sometimes called al-Basri, after his birthplace in the city of Basra. He was also nicknamed Ptolemaeus Secundus (”Ptolemy the Second”) or simply “The Physicist” in medieval Europe.
Born circa 965, in Basra, Iraq and part of Buyid Persia at that time, he lived mainly in Cairo, Egypt, dying there at age 76. Over-confident about practical application of his mathematical knowledge, he assumed that he could regulate the floods of the Nile. After being ordered by Al-Hakim bi-Amr Allah, the sixth ruler of the Fatimid caliphate, to carry out this operation, he quickly perceived the impossibility of what he was attempting to do, and retired from engineering. Fearing for his life, he feigned madness and was placed under house arrest, during and after which he devoted himself to his scientific work until his death.
Ibn al-Haytham is regarded as the “father of modern optics” for his influential Book of Optics (written while he was under house arrest), which proved the intromission theory of vision and refined it into essentially its modern form. He is also recognized so for his experiments on optics, including experiments on lenses, mirrors, refraction, reflection, and the dispersion of light into its constituent colours. He studied binocular vision and the Moon illusion, described the finite speed of light, and argued that it is made of particles travelling in straight lines. Due to his formulation of a modern quantitative and empirical approach to physics and science, he is considered the pioneer of the modern scientific method and the originator of the experimental nature of physics and science. Author Bradley Steffens describes him as the “first scientist”. He is also considered by A. I. Sabra to be the founder of experimental psychology for his approach to visual perception and optical illusions, and a pioneer of the philosophical field of phenomenology or the study of consciousness from a first-person perspective. His Book of Optics has been ranked with Isaac Newton’s Philosophiae Naturalis Principia Mathematica as one of the most influential books in the history of physics, for starting a revolution in optics and visual perception.
Ibn al-Haytham’s achievements include many advances in physics and mathematics. He gave the first clear description and correct analysis of the camera obscura.
In his experiments, Ibn Al-Haitham used the term “Al-Bayt al-Muthlim, translated in English as dark room. In the experiment he undertook, in order to establish that light travels in time and with speed, he says: “If the hole was covered with a curtain and the curtain was taken off, the light travelling from the hole to the opposite wall will consume time.” He reiterated the same experience when he established that light travels in straight lines. A revealing experiment introduced the camera obscura in studies of the half-moon shape of the sun’s image during eclipses which he observed on the wall opposite a small hole made in the window shutters. In his famous essay “On the form of the Eclipse” (Maqalah-fi-Surat-al-Kosuf) he commented on his observation “The image of the sun at the time of the eclipse, unless it is total, demonstrates that when its light passes through a narrow, round hole and is cast on a plane opposite to the hole it takes on the form of a moon-sickle”.
In his experiment of the sun light he extended his observation of the penetration of light through the pinhole to conclude that when the sun light reaches and penetrates the hole it makes a conic shape at the points meeting at the pinhole, forming later another conic shape reverse to the first one on the opposite wall in the dark room. This happens when sun light diverges from point “ﺍ” until it reaches an aperture and is projected through it onto a screen at the luminous spot. Since the distance between the aperture and the screen is insignificant in comparison to the distance between the aperture and the sun, the divergence of sunlight after going through the aperture should be insignificant. In other words, should be about equal to. However, it is observed to be much greater when the paths of the rays which form the extremities of are retraced in the reverse direction, it is found that they meet at a point outside the aperture and then diverge again toward the sun. This an early accurate description of the Camera Obscura phenomenon.

Alhacen’s observations of light’s behaviour through a pinhole. With a second hole the image is doubled.
Light generally travels in a straight line. When rays reflected from a bright subject pass through the small hole in thin material they do not scatter but cross and reform as an upside down image on a flat white surface held parallel to the surface through which the hole has been pierced. Ibn Al-Haitham established that the smaller the hole is, the clearer the picture is.