This is the third article of a series that will be focused on influential Muslim scientists. To see if any other articles have been uploaded, search the tag or keywords ‘Muslim scientists’
Abu Ali al-Hasan ibn al-Hasan ibn al-Haytham, known as Alhazen in Latin, is among the greatest scientists of physics, mathematics, and astronomy in the Medieval period. He was born in 965 AD in Basra, Iraq and died in 1040 AD in Cairo after making significant contributions in various branches of science. He is credited with significant contributions to the study of motion in which he made discovery of principle of inertia. He made contributions in celestial physics and the science of statics but he performed pioneering research in optics and made it into a new science and laid foundations for future development of the subject.
He is credited with developing the method of experimental verification of theories. He would not be satisfied with just theorems but would perform experiments to know the correct solutions. The modern scientific method of experimental studies finds its roots in Ibn al-Haytham’s methodology of studies. “He combined elaborate mathematical treatment with well conceived physical models and careful experimentation. Like Archimedes, he was both a theoretical and experimental physicist. He made experiments to determine the rectilinear motion of light, the properties of shadows, the use of lens, the camera obscura, that he studied mathematically for the first time, and many other essential optical phenomena. He even had a lathe on which he made curved lenses and mirrors for his experiments.” 
His monumental book on optics “Kitab al-Manazir” (The Book of Optics) paved the way for scientific study of optics as a separate branch of physical sciences. For his fundamental work in optics, he is regarded as “Father of Optics” by many scholars. The “Book of Optics” comprises of seven volumes and has been ranked alongside Isaac Newton’s “Philosophiae Naturalis Principia Mathematica” as one of the most influential books ever written in physics, which drastically transformed the understanding of light and vision.
In his great book he correctly explained and proved the modern intromission theory of visual perception. He proved that Euclidean and Ptolemaic theories of visual rays issuing from the eye are not correct and he regarded eye as a passive receptor of light. His work on optics form is the first major advance after Euclid and Ptolemy, and in visual physiology after Gelen. The world did not see such extraordinary contribution in optics until Newton and Kepler emerged on the horizon and explained further the nature of light , and Helmholtz in 19th Century made advances in understanding of visual perception. His influence on the subject was immense and pervasive and was well recognized into the 18th and 19th century. His idea of point-to-point projection of the visual world into the brain forms one of the bases of the modern visual physiology. He made an in depth examination of the passage of light through different types of media and discovered the laws of refraction.
“He was convinced that the adequate theory of vision must combine Euclid and Ptolemy ‘mathematical’ approach and ‘physical’ doctrine of the naturalists. The result of his reflections in the paper ‘Optics’, supported by the experimental approach, is the new theory of vision, much richer and perfected than any before. He thought that light and color, two physical features that exist independently from the observed subject, in straight lines originates from the each point of visible object. Al-Haitam concludes that what we perceive is actually the object which is on a certain distance from the eye and which have certain shape and size, and vision itself is the result of intervention by the visual material received by the brain and stored information from previous experiences.” 
He performed study of spherical and parabolical mirrors and spherical aberration in catoptrics. He observed that the ratio between the angle of incidence and refraction does not remain constant and he made investigations on the magnifying power of a lens. He realized that in a parabolical mirror all the rays are concentrated at one point so that it the best type of burning mirror. His study on the catoptrics also contains what it is known as “Alhazen’s Problem.” The “Alhazen’s Problem” is connected with reflection from a spherical surface: from two points in a plane of a circle to draw lines meeting at a point at the circumference and making equal angles with the normal at that point. Its main application in optics is to solve the problem like when there is a given light source and a spherical mirror, one has to find the point on the mirror where the light will be reflected to the eye of an observer. This leads to a fourth degree equation which he solved by the interaction of a hyperbola and circle.
With respect to refraction he made outstanding contributions. He applied the rectangle of velocities at the surface of refraction, and made careful experiments by placing a graduated cylinder in water to measure the angle of refraction. He also made study of refraction through glass cylinders and spheres, and made efforts to arrive at the magnifying effect of the plano-convex lens.
In the field of atmospheric phenomena with respect to refraction he made significant contributions as well. He calculated the amount of atmospheric refraction by measuring the distance of a fixed star from the pole at the time of rising and at zenith with help of an armilla. He made interesting observations with respect to the twilight, dawn, and changes in the sizes of the sun and moon. He calculated that twilight ends when the Sun is 19 degrees below the horizon. He also explained the rainbow on the principle of refraction. In short, this Muslim scientist is responsible for some of the most amazing discoveries in history that have contributed to many subjects of science, including but not limited to physics, astronomy, and mathematics.
1. “Science and Civilization in Islam” by Seyyed Hossein Nasr
2. “Ibn al-Haitham–Father of Optics and Describer of Vision Theory” by Izet Masic, Cathedra for Family Medicine, Faculty of Medicine of Sarajevo University, B&H