Optics

Optics is the branch of physics concerned with the behaviour and properties of light, including its interactions with matter and the design of instruments that detect or manipulate it. Traditionally, optics focuses on visible, ultraviolet and infrared light, though all electromagnetic radiation—from radio waves to X-rays—exhibits similar fundamental properties. Many optical phenomena can be described within the framework of classical electromagnetism, but simplified models are often employed in practice.
The two principal classical models used are:

• Geometric optics, which treats light as rays travelling in straight lines and bending at interfaces by reflection and refraction.
• Physical optics, which incorporates wave effects such as diffraction and interference that geometric optics cannot explain.

Advances in nineteenth-century electromagnetic theory revealed that light is a form of electromagnetic radiation. Some optical phenomena, however, require quantum explanations based on wave–particle duality, modelling light as photons. Quantum optics applies quantum mechanics to light–matter interactions.
Optics underpins numerous scientific and engineering disciplines, including astronomy, electrical engineering, photography, medicine (especially ophthalmology and optometry) and materials science. Its practical applications appear in everyday devices such as mirrors, lenses, microscopes, telescopes, lasers and fibre-optic communication systems.

History

Optics originated with early lens-making by ancient civilizations. Egyptian and Mesopotamian artisans crafted polished crystal lenses as early as 2000 BC, a tradition echoed in later Greek and Roman practices using glass spheres filled with water. These practical developments were accompanied by early theories of light and vision.
Ancient Greek thinkers proposed two competing models of vision:

• The intromission theory, associated with Democritus, Epicurus and Aristotle, suggested that objects emitted image-bearing effluences (eidola) that entered the eyes.
• The emission theory, articulated by Plato, held that the eyes emitted rays that interacted with objects. Plato also noted mirror reversals in his Timaeus.

In the 3rd–4th centuries BC, Euclid developed geometrical optics, linking vision with geometry and describing rules of perspective, mirror reflections and qualitative refraction. Ptolemy later expanded on these ideas, proposing a combined extramission–intromission theory and attempting empirical measurements of refraction.
Writers of the early Christian and Roman periods, such as Plutarch, described multiple reflections from curved mirrors and the formation of real and virtual images. By late antiquity, Greek optical knowledge had declined in influence.
During the Middle Ages, optical science was revitalised within the Islamic world. Al-Kindi evaluated Greek theories, favouring emission models for their quantifiable qualities. In 984, the mathematician Ibn Sahl derived a correct law of refraction—equivalent to Snell’s law—and used it to design focusing mirrors and lenses.
In the 11th century, Ibn al-Haytham (Alhazen) wrote the seminal Book of Optics. He rejected emission theories entirely and proposed that light travels in straight lines from objects to the eye, explaining vision through reflected and refracted rays. He supported his ideas with systematic experiments, shaping the empirical foundations of optical science. Translated into Latin in the 1200s and synthesised by scholars such as Witelo, Alhazen’s work dominated European optics for centuries.
In medieval Europe, thinkers such as Robert Grosseteste and Roger Bacon advanced optical theory based on Aristotelian and Platonic principles, while experimenting with lenses. The invention of wearable eyeglasses in Italy around 1286 spurred an industry of lens-grinding in Venice, Florence, the Netherlands and German states. Practical mastery of lenses enabled the subsequent invention of the compound microscope (c. 1595) and the refracting telescope (1608).
The seventeenth century saw major theoretical advances. Johannes Kepler refined geometrical optics, correctly identifying the retina as the organ receiving images and explaining the functioning of lenses, mirrors and pinhole cameras. He also described the Keplerian telescope, which used two convex lenses to achieve high magnification.
Around the same period, René Descartes proposed a mechanical theory of light in Le Monde, treating light as a pressure transmitted through a medium. Although influential, this theory departed significantly from ancient emission concepts.
In the late 1600s, Isaac Newton formulated the corpuscular theory of light, showing through prism experiments that white light is composed of different colours separable into a spectrum. His particle-based model successfully explained many optical effects but struggled with diffraction and interference.
Subsequent developments in the eighteenth and nineteenth centuries—including the wave theories of Huygens and Young, and Maxwell’s electromagnetic equations—solidified the wave nature of light. Experimental discoveries in the early twentieth century introduced the photon concept, giving rise to modern quantum optics.