Light
Waves That Paint Reality
Electromagnetic Waves
Light consists of photons - quantized excitations of electromagnetic field. Classically, these excitations propagate as electromagnetic waves: a changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. The two fields sustain each other in a loop that races through spacetime without needing any medium to carry it. James Clerk Maxwell realized this in 1865, unifying electricity, magnetism, and optics into a single framework. Every color you see, every bit of warmth from a fire, every radio signal reaching your phone is this same phenomenon at different frequencies.
Individual photons and their quantum behavior are covered on the Photon page. The underlying electromagnetic field that gives rise to them is its own topic. This page focuses on something different: how light behaves as a wave traveling through space and interacting with matter. Refraction, diffraction, polarization, scattering. These wave behaviors shape nearly everything you see.
Refraction
Drop a straw into a glass of water. It looks bent at surface. The straw is straight - but light reaching your eyes from below waterline has changed direction, shifting where the submerged portion appears to be. Light genuinely changes direction when it crosses from one material into another. This happens because light travels at different speeds in different media. In vacuum, light moves at its maximum speed. In glass, it slows to roughly two-thirds of that. In water, about three-quarters.
When a wave front hits an interface at an angle, one side enters slower medium before the other. That side slows down first while opposite side still travels at original speed. Result is the wavefront pivots, like a marching band turning a corner when one end of a row steps onto sand. Snell's law quantifies this: ratio of sines of incoming and outgoing angles equals ratio of wave speeds in two media. A higher refractive index means slower light and sharper bending.
Refraction is why lenses work. A curved glass surface bends light rays by different amounts at different points, converging them to a focus. Your eyes use this principle with a flexible lens that changes shape to focus on objects at different distances. Telescopes, microscopes, cameras, eyeglasses, fiber optic cables. All rely on controlling how light bends when it crosses boundaries between materials.
Diffraction
Waves do not travel in perfectly straight lines. When light passes through a narrow opening or around an edge, it spreads out. This is diffraction. If you look at a distant streetlight through a window screen, you see a cross-shaped pattern of bright spots. Light is bending around mesh wires and interfering with itself.
Thomas Young's double-slit experiment in 1801 demonstrated this powerfully. Light passing through two narrow slits creates an interference pattern on a screen behind them. Bright bands where wave crests align and reinforce each other. Dark bands where crests meet troughs and cancel. This was definitive proof of wavelike behavior in light. Diffraction becomes most noticeable when opening size is comparable to wavelength of light. For visible light, that means openings on order of hundreds of nanometers. This is also why there is a fundamental resolution limit to optical microscopes. You cannot resolve details smaller than roughly half a wavelength of light used to observe them.
Polarization
Electromagnetic waves oscillate. Electric field swings in one direction, magnetic field swings perpendicular to it, and both are perpendicular to direction of travel. But which direction does electric field swing? In ordinary light from sun or a light bulb, it swings in all directions randomly. Every instant, oscillation direction is different. This is unpolarized light.
A polarizing filter acts like a picket fence for waves. Only oscillations aligned with fence slats pass through. Everything else is absorbed. Put on polarized sunglasses and glare from a wet road vanishes. Light reflecting off flat surfaces becomes partially polarized in horizontal direction. Vertically oriented filter in your sunglasses blocks that horizontal component, cutting glare while allowing other light through.
Cross two polarizing filters at right angles and no light passes through at all. First filter selects vertical oscillations. Second filter only passes horizontal ones. Nothing survives both. LCD screens exploit this principle. Two crossed polarizers with a liquid crystal layer between them that can rotate polarization direction on command, pixel by pixel. Most LCD screens you look at use polarization to create images.
Why Sky Is Blue
Sunlight contains all visible wavelengths. When it enters atmosphere, it encounters nitrogen and oxygen molecules far smaller than wavelength of visible light. These tiny molecules scatter light, but not equally. Scattering intensity is inversely proportional to fourth power of wavelength. Blue light, with its shorter wavelength, scatters roughly ten times more than red light. This is Rayleigh scattering.
Look up during daytime and you see blue because shorter-wavelength photons, scattered far more strongly, reach your eyes from every direction across sky. Look toward horizon at sunset and you see red and orange. Sunlight at sunset travels through much more atmosphere to reach you. Most blue light has already been scattered away in other directions. What remains is longer-wavelength red and orange. Same physics, different path length, completely different color. Mars has reddish sky during day and blue sunsets, an inversion caused by different sized dust particles in its thin atmosphere scattering light differently than Earth's molecular atmosphere.
Rainbows and Dispersion
Refraction depends on wavelength. Blue light bends more than red when entering glass or water because shorter wavelengths interact more strongly with electrons in medium, slowing down more. This wavelength-dependent bending is dispersion. Newton demonstrated it by passing white sunlight through a glass prism. What emerged was not white but a continuous band of colors from red to violet. White light is a mixture. Prism separates it.
Rainbows form by same principle, but inside water droplets. Sunlight enters a raindrop, refracts at front surface, reflects off back surface, and refracts again as it exits. Each wavelength bends by a slightly different angle. Red light exits at roughly 42 degrees from incoming direction. Violet exits at roughly 40 degrees. Millions of droplets at different positions in sky each contribute one color at correct angle to your eye, assembling a full arc.
Double rainbows occur when light reflects twice inside each droplet before exiting. Second reflection reverses color order and loses some intensity. Secondary arc appears above primary one, fainter and with red on inside rather than outside. Region between two arcs appears noticeably darker because no light is directed into that angular range. This dark band is called Alexander's band, after Alexander of Aphrodisias who first described it around 200 CE.
Universal Speed Limit
Light in vacuum travels at exactly 299,792,458 meters per second. This number is not just a property of light. It is built into structure of spacetime itself. Einstein showed that nothing carrying information or energy can exceed this speed. It is speed of causality, the maximum rate at which one event can influence another. Massless particles like photons must travel at exactly this speed - no faster, no slower. This speed limit is what welds space and time into a unified geometry.
When light enters a medium like glass or water, its effective speed drops. A common explanation claims photons are absorbed and re-emitted by atoms, with tiny delays between each hop. But if that were true, re-emitted photons would scatter in random directions and light could not maintain coherence through glass. What actually happens: incoming electromagnetic wave drives oscillations in electrons of the medium. These oscillating charges radiate their own electromagnetic waves. Superposition of original wave and this forward-scattered radiation produces a combined wave with a slower phase velocity. Coherence and direction are preserved because forward scattering adds constructively, while scattering in other directions largely cancels. Refractive index of a material quantifies the slowdown: ratio of light speed in vacuum to effective wave speed in that material.
Slowing and Stopping Light
Light's speed in vacuum is absolute. Its effective speed inside matter is surprisingly flexible. In 1999, physicist Lene Hau slowed a pulse of light to 17 meters per second – bicycle speed – by sending it through a cloud of sodium atoms cooled to a few billionths of a degree above absolute zero. The pulse, a kilometer long in free space, compressed to just a few millimeters as it passed through the cloud, dragged down to a crawl by the way the atoms collectively interacted with it.
Two years later her team did something stranger. They switched off the auxiliary laser that was making the cloud transparent, and the light pulse vanished. Its quantum information did not, though. It was stored inside the cloud as a coherent pattern of atomic states, like writing a song onto a tape and shelving it. Switch the laser back on and the pulse reappeared, resuming its journey as if nothing had happened.
The original photon ceased to exist when the laser was off. Its quantum state lived on as an excitation of matter. When the laser came back, a new photon was created from that stored pattern, carrying the same quantum information forward. Light and matter, the experiment showed, are interconvertible at the quantum level – consistent with the picture of all particles as field excitations rather than permanent objects. This is the basis of quantum memory: photonic information briefly held in atomic ensembles, then released again. A photon's quantum state can be parked, retrieved, and even teleported between systems that never directly meet.
