History of Optics

Optics, according to the Kubbealtı Lügatı Sözlüğü, is defined as "the branch of physics that studies light and phenomena related to light" . The earliest concepts of light were rooted in religious origins. For example, in Ancient Egypt, light was regarded as a divine power emanating from the eye of the god sun. Similarly, in the Bible, light is described as a created entity brought into existence by God on the first day of creation. In these sacred texts, light is associated with goodness, while darkness is linked to evil. The perception of light as a divine entity reflects the fact that ancient societies were more concerned with religious and symbolic meanings than with scientific explanations.

Ancient Greek philosophers initiated studies to understand the nature of light and the mechanism of vision. The Pythagoreans believed that vision occurred through something emitted from the eye toward the object, while followers of Democritus argued that a solid substance traveled from the object to the eye. Followers of Empedocles, however, maintained that vision resulted from a movement in both directions: from the eye to the object and from the object to the eye.

Optics in the Medieval Islamic World

In the medieval period, Ibn al-Haytham (known in Latin as Alhazen) conducted significant experimental studies on the nature of light and the mechanism of vision. Ibn al-Haytham demonstrated that vision does not occur through rays emitted from the eye, as previously believed, but through light rays entering the eye from objects. He based this conclusion on observations of the discomfort felt when looking directly at bright objects and the lingering afterimages that follow such exposure. He also examined the anatomy of the eye and proposed that vision occurs not on the surface of the crystalline lens, as ancient thinkers had claimed, but on the retina. His work diverged from ancient Greek thought by relying on empirical observation, yet in the West, the Greek concept of rays emanating from the eye continued to exert influence long duration.

Ibn al-Haytham (Generated by Artificial Intelligence.)

At the center of Ibn al-Haytham’s optical corpus is the Kitāb al-Manāzir, organized into seven “books” or volumes: the first three focus on direct vision, the fourth to sixth on reflection, and the seventh on refraction. Its circulation in the Latin world began in the late 12th and early 13th centuries; Friedrich Risner’s 1572 Basel edition (Opticae Thesaurus) had a foundational influence on European scholars including Roger Bacon, John Pecham, Witelo, Kepler, Snell, Fermat, and Descartes. In the Eastern tradition, Kamāl al-Dīn al-Fārisī’s Tenkīḥ al-Manāzir and Taqiyy al-Dīn’s Kitāb-i Nūr show deep engagement with his work. Within this framework, the treatise On Light completes the Kitāb al-Manāzir by elaborating on themes such as the propagation of light, transparency and opacity, and “secondary light” (the faint light emitted by illuminated objects). The corpus also includes independent treatises on specific topics such as parabolic and spherical mirrors, lenses, rainbows and halos, eclipses, shadows, and starlight.

Theory of Vision: Ibn al-Haytham criticized the ancient notion of “emission of rays from the eye” (the Emission Theory) and argued that vision occurs through the effect of light and color entering the eye from objects. The core of his argument is this: even if rays were emitted from the eye, no vision would occur unless something traveled from the object to the eye; the pain experienced when looking at bright objects further demonstrates that the eye is a passive receiver. Vision is established by a cone of light with its source at the object and its apex at the eye; the distance or proximity of the object alters the angle of the cone reaching the eye, thereby determining the perception of size.

Reflection: Through experiments, Ibn al-Haytham examined the behavior of primary and secondary light rays on flat, spherical, cylindrical, and conical mirrors. He provided an original geometric foundation for the principle that angle of incidence equals angle of reflection. This approach, known as the “Quadrilateral of Speeds,” is based on decomposing the incident motion into components perpendicular and tangential to the surface. The surface’s “resistance” reverses the perpendicular component while preserving the tangential one, forcing the resultant motion to reflect at an equal angle to the normal. This method also provided a systematic basis for classifying optical aberrations observed in mirrors of varying curvature.

Refraction: The same component analysis is applied to refraction: when light strikes the interface between two media at an angle, it encounters resistance dependent on density. When moving from a less dense to a denser medium, light bends toward the normal; in the reverse case, it bends beyond the normal. Ibn al-Haytham explained this using mechanical analogies—for example, a stone dropped perpendicularly penetrates a plate more easily than one striking at an angle. He formulated the principle that “perpendicular motion is stronger, and motion closer to perpendicular is easier than motion farther from it,” thereby constructing the geometry of refraction through a velocity-component scheme.

Camera Obscura

The camera obscura, the earliest known form of the photographic device, was one of Ibn al-Haytham’s most important tools in optical experiments. In this apparatus, a small aperture is made in one wall of a light-tight box; light reflected from external objects passes through the aperture and projects an inverted image onto the inner surface. As the aperture becomes smaller, the image becomes sharper but less bright; thus, the use of lenses can enhance image clarity.

Ibn al-Haytham observed that during a solar eclipse, light entering a room through a small hole in a window projected a crescent-shaped image on the wall, thereby demonstrating experimentally that light travels in straight lines. He referred to this device as “bayt al-muzlim” (Arabic: dark room). The concept was later translated into Latin as camera obscura and formed the theoretical foundation of photographic technology.

Ibn al-Haytham’s Camera Obscura Illustration (Generated by Artificial Intelligence.)

Renaissance and Modern Optics

By the end of the 13th century, Italian lensmakers invented convex lenses to correct presbyopia (farsightedness caused by aging). However, this invention did not generate scientific interest at the time, and lenses were regarded as magical objects. Centuries later, Kepler examined how light rays are refracted through lenses and explained how eyeglasses correct vision impairments. Kepler claimed that vision occurs via an inverted image formed on the retina. Leonardo da Vinci had previously described a similar mechanism by comparing the eye to a camera obscura.

By the end of the 14th century, Dutch lensmakers combined two lenses to invent the telescope. Galileo was the first to recognize its scientific potential difference, and by improving its design, he studied the planets. He discovered the four moons of Jupiter and observed the phases of Venus. Galileo’s observations supported Copernicus’s heliocentric universe model. Kepler, in turn, developed the theoretical foundation for these discoveries, explaining the principles of light refraction and telescope design work. During this period, a new scientific methodology based on observation began to emerge, prioritizing empirical verification over the speculative approaches of earlier eras.

As a result of these scientific approaches, significant insights were gained into the nature of light and vision. Light is regarded as a substance traveling from every point of an object toward the observer’s eye. Objects may either be direct sources of light or reveal their shapes and colors through reflected light. Light rays pass through the pupil and form an image on the retina, which is then transmitted to the brain.