Optics
Optics is the branch of physics that studies how electromagnetic radiation behaves, how it can be manipulated, and how it is detected. The word itself comes from the ancient Greek optike. The earliest known lenses date from as early as 2000 BC, polished from crystal, often quartz, and found on Crete. From that small disc of crystal to the laser scanning a barcode at a supermarket checkout, the same questions keep returning. Does vision come from the eye reaching out, or from the world entering in? Is light a ray, a wave, or a particle? Why does a prism split white into colour, and why is the sky blue? This is the story of how a handful of philosophers, monks, and physicists turned a polished stone into a science that now carries the Internet.
Plato first articulated the emission theory, the idea that the eyes send out rays to accomplish sight. He even noted in Timaeus how a mirror reverses left and right. The opposing camp, the intromission theory, held that objects cast off copies of themselves, called eidola, which the eye caught. Democritus, Epicurus, and Aristotle stood among its propagators, yet it remained speculation without experimental foundation.
Euclid, in the 4th to 3rd century BC, wrote a treatise titled Optics and tied vision to geometry. He built on Plato's emission theory, set out the rules of perspective, and stated the principle that light takes the shortest path. He still doubted that a beam from the eye could light up the stars the instant someone blinked. Ptolemy later held a blended extramission-intromission view, picturing a cone of rays whose vertex sat within the eye and whose base mapped the visual field.
Alhazen, writing the Book of Optics, the Kitab al-manazir, in the early 11th century, overturned the older picture. He rejected rays emitted by the eye and argued instead that light reflects in straight lines from every point of an object and then enters the eye. He could not yet explain how the eye captured those rays. His work was largely ignored in the Arabic world. Around 1200 it was anonymously translated into Latin and expanded by the Polish monk Witelo, becoming the standard text on optics in Europe for the next 400 years.
The first wearable eyeglasses appeared in Italy around 1286, launching an industry of grinding and polishing lenses. Venice and Florence led in the thirteenth century, with later centres in the Netherlands and Germany. Spectacle makers improved their lenses through trial and observation rather than theory, since the optical theory of the day could not even explain how spectacles worked.
Roger Bacon, the most famous disciple of the English bishop Robert Grosseteste, used parts of glass spheres as magnifying glasses. With them he showed that light reflects from objects rather than being released by them. Grosseteste himself had approached light from four angles at once: as epistemology, as metaphysics, as physics, and as theology.
The compound optical microscope arrived around 1595 and the refracting telescope in 1608, both emerging from the spectacle-making centres of the Netherlands. Johannes Kepler then supplied the theory the craftsmen had lacked. In the early 17th century he worked out lenses, pinhole cameras, and the inverse-square law of light intensity. He deduced that the retina is the organ that records images, and he described an improved instrument, the Keplerian telescope, using two convex lenses for higher magnification.
René Descartes, in the mid-17th century, explained reflection and refraction by assuming light was emitted by the objects that produced it. In the late 1660s and early 1670s, Isaac Newton extended these ideas into a corpuscle theory and showed with a prism that white light is a mixture of colours. In 1690 Christiaan Huygens proposed a wave theory, building on suggestions Robert Hooke had made in 1664. Hooke publicly attacked Newton's views on light, and the feud lasted until Hooke died. When Newton published Opticks in 1704, his standing in other areas of physics led most to count him the winner.
Newton's account held until the early 19th century, when Thomas Young and Augustin-Jean Fresnel ran experiments on interference that established light's wave nature. Young's double slit experiment showed light obeying the superposition principle, a wave property the corpuscle theory could not predict. James Clerk Maxwell unified wave optics with electromagnetic theory in the 1860s, and his equations predicted electromagnetic waves in 1865.
Max Planck reopened the question in 1899 by modelling blackbody radiation, assuming energy passed between light and matter only in discrete amounts he called quanta. In 1905 Albert Einstein's theory of the photoelectric effect established the quantization of light itself. In 1913 Niels Bohr showed atoms emit only discrete amounts of energy, explaining the sharp lines in spectra. These insights fed both quantum optics and quantum mechanics as a whole, culminating in quantum electrodynamics.
Geometrical optics treats light as rays travelling in straight lines, bending at the boundaries between media according to the laws of reflection and refraction. Those laws were known empirically as far back as 984 AD. They can be derived from Fermat's principle, which holds that a ray follows the path it can traverse in the least time.
Specular reflection gives the predictable gloss of a mirror, while diffuse reflection from paper or rock can only be described statistically. Many diffuse surfaces follow Lambert's cosine law, appearing equally bright from any angle. In specular reflection the angle to the surface normal is the same for the incoming and outgoing rays, and both lie with the normal in one plane.
Flat mirrors place an image the same distance behind the glass as the object stands in front, the same size, and parity inverted, which we read as a left-right flip. Corner reflectors send rays straight back the way they came, an effect called retroreflection. Parabolic mirrors bring parallel rays to a common focus, while spherical mirrors smear that focus through spherical aberration. An upright mirror image is always virtual, but an inverted one is real and can be cast onto a screen.
Snell's Law describes how a ray bends when it crosses from one index of refraction to another, and that bending is what makes lenses possible. In 984 the Persian mathematician Ibn Sahl wrote a treatise on burning mirrors and lenses that correctly stated a law of refraction equivalent to Snell's, using it to compute optimum lens and mirror shapes. In most materials the index varies with frequency, an effect called dispersion, which is why a prism spreads light into a spectrum.
Gradient-index materials, whose index changes gradually with position, curve light rays along smooth paths. A change of index with height in hot air bends rays to create mirages, the shimmering false pools seen on a hot road. When light passes from a high index toward a low one at a steep angle, none is transmitted and all is reflected. This total internal reflection lets an optical fibre carry light down its length with essentially nothing lost.
A lens is rated by its focal length, positive for a converging lens and negative for a diverging one, with shorter focal lengths giving a stronger effect. Converging lenses bring parallel rays to a rear focal point one focal length away. Diverging lenses spread them so they seem to start at a front focal point. Lenses also suffer aberrations: monochromatic ones from imperfect geometry, and chromatic ones because the index varies with wavelength.
Diffraction is the most common way light interference shows itself. Francesco Maria Grimaldi first described it in 1665 and coined the term from the Latin diffringere. Later that century Robert Hooke and Isaac Newton described diffraction in Newton's rings, and James Gregory recorded the patterns cast by bird feathers. In 1803 Thomas Young built the first physical-optics model of diffraction from his two-slit experiments, and Fresnel firmed up the mathematics in 1815 and 1818.
When two waves of the same wavelength meet in phase, crests align with crests and the light brightens, constructive interference. Out of phase, crests meet troughs and the light dims, destructive interference. Antireflective coatings exploit this, using a single layer one-fourth of a wavelength thick to cancel reflections near 550 nm at the centre of the visible spectrum. Constructive interference in thin films instead builds strong reflections, the same effect that paints rainbows across an oil slick.
Diffraction also sets a hard limit on resolution. Light through an aperture forms an Airy pattern, a central Airy disk ringed by fainter circles, and two sources closer than that disk cannot be told apart. Lord Rayleigh's criterion marks the threshold where they count as resolved. A larger aperture gives finer detail, which is why interferometry, mimicking enormous baselines, reaches the greatest angular resolution. For starlight, the atmosphere blurs the view through scattering, the twinkling astronomers call seeing, now countered by adaptive optics.
The first working laser was demonstrated on the 16th of May 1960 by Theodore Maiman at Hughes Research Laboratories, and was first dismissed as a solution looking for a problem. Its earliest appearance in everyday life was the supermarket barcode scanner, introduced in 1974. The laserdisc player of 1978 was the first consumer product to carry a laser, but the compact disc player, common in homes from 1982, made the technology truly familiar. These discs are read by a semiconductor laser less than a millimetre wide.
The human eye focuses light through the cornea, the pupil, and the lens onto the retina, where rods and cones sit. Rod cells handle black-and-white and peripheral vision and outnumber cone cells twenty to one. Cone cells crowd the fovea, come in three frequency-tuned varieties, and carry colour and fine detail. Ciliary muscles reshape the lens to focus, a process called accommodation, with a normal near point around 25 cm. As the lens stiffens with age the near point recedes, the condition known as presbyopia.
The atmosphere stages its own optics. Rayleigh scattering redirects blue sunlight to make the sky blue and reddens the sun at sunrise and sunset. A single internal reflection inside raindrops makes a rainbow spanning 40 to 42 degrees with red on the outside, while a double reflection adds a second bow from 50.5 to 54 degrees with the colours reversed. The same crystals that scatter light into halos and sun dogs hint at how far this science still reaches, from a polished quartz lens on Crete to the fibre that now carries the world's words at the speed of light.
Common questions
What is optics in physics?
Optics is the branch of physics that studies the behaviour, manipulation, and detection of electromagnetic radiation and its interactions with matter. It usually describes visible, ultraviolet, and infrared light, but extends to radio waves, microwaves, and X-rays.
Who wrote the Book of Optics?
Alhazen, also known as Ibn al-Haytham, wrote the Book of Optics, the Kitab al-manazir, in the early 11th century. He rejected the idea that the eye emits rays and argued that light reflects in straight lines from objects into the eye.
Who invented the first working laser and when?
Theodore Maiman demonstrated the first working laser on the 16th of May 1960 at Hughes Research Laboratories. The laser was first described as a solution looking for a problem before becoming a multibillion-dollar industry.
What is the difference between geometrical optics and physical optics?
Geometrical optics treats light as rays travelling in straight lines that bend at surfaces through reflection and refraction. Physical optics treats light as an electromagnetic wave and explains effects such as interference and diffraction that geometric optics cannot.
Why is the sky blue according to optics?
The sky is blue because of Rayleigh scattering, which redirects higher-frequency blue sunlight back toward the observer. Because blue light scatters more easily than red, the sun looks reddish through a thick atmosphere at sunrise and sunset.
How does total internal reflection make fibre optics work?
Total internal reflection occurs when light travelling from a high index of refraction toward a low one strikes the boundary at a steep angle, so all the light is reflected and none is transmitted. As light travels down an optical fibre it undergoes this reflection, allowing essentially no light to be lost over the length of the cable.
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