Standard physics glossary

optics · intro

Snell's Law

A law of optics describing the relationship between the angles of incidence and refraction when a wave, typically light, passes between two media with different refractive indices. It states that the refractive index of the first medium multiplied by the sine of the incidence angle equals the refractive index of the second medium multiplied by the sine of the refraction angle. Named after Dutch astronomer Willebrord Snellius, it is the foundational equation for designing lenses and other refractive optical components.

astro · intro

Solar Wind

A continuous stream of charged particles, mainly protons and electrons, ejected from the Sun's outer atmosphere, the corona, and traveling outward through the solar system at speeds typically between 300 and 800 kilometers per second. The solar wind interacts with planetary magnetic fields, compressing and shaping them, and its interaction with Earth's magnetosphere produces auroras near the polar regions. Strong bursts of solar wind can also disrupt satellites and power grids.

waves · intro

Sound Wave

A longitudinal mechanical wave created by vibrating objects, traveling through a medium such as air, water, or a solid, as alternating regions of compression and rarefaction of the medium's particles. Sound waves require a physical medium to travel through and cannot propagate through a vacuum, unlike electromagnetic waves. The frequency of a sound wave determines its pitch, while its amplitude determines its loudness.

relativity · intro

Spacetime

A unified four-dimensional framework combining the three familiar spatial dimensions with time into a single continuum, introduced formally in special relativity and central to general relativity. In spacetime, events are located not just by where they occur but also by when, and the separation between two events depends on the observer's motion in a way that mixes space and time coordinates. Massive objects curve spacetime, and this curvature is what we perceive as gravity.

relativity · intermediate

Spacetime Curvature

The warping of the four-dimensional fabric of spacetime caused by the presence of mass and energy, as described by Einstein's general theory of relativity. Rather than gravity being a force pulling objects together, general relativity explains that massive objects like stars and planets curve the spacetime around them, and other objects simply follow the straightest possible paths, called geodesics, through this curved geometry, which appears to us as gravitational attraction.

cosmology · intermediate

SPARC Database

Also called: SPARC

A curated collection of high-quality rotation curve data, whose name stands for Spitzer Photometry and Accurate Rotation Curves, for a large sample of nearby spiral and irregular galaxies, combining infrared photometry with radio observations of gas rotation speeds. The SPARC database is widely used by researchers testing predictions of dark matter distribution models and alternative gravitational theories against observed galaxy rotation curves.

relativity · intro

Special Relativity

A theory developed by Albert Einstein in 1905 that describes the physics of objects moving at constant velocity, particularly at speeds approaching the speed of light, based on two postulates: that the laws of physics are the same in all inertial reference frames, and that the speed of light in a vacuum is constant for all observers regardless of their motion. Special relativity leads to phenomena including time dilation, length contraction, and the equivalence of mass and energy.

thermo · intro

Specific Heat

The amount of heat required to raise the temperature of one unit of mass of a substance by one degree, a characteristic property that varies from one material to another. Water has an unusually high specific heat compared to most substances, meaning it absorbs a large amount of heat with only a small temperature change, which is why it is effective at moderating temperature in cooling systems and coastal climates.

quantum · intermediate

Spectral Line

A narrow, distinct feature appearing at a specific wavelength within the spectrum of light emitted or absorbed by a substance, corresponding to a transition between two discrete quantum energy levels in an atom or molecule. Emission lines appear bright against a darker background when atoms release light, while absorption lines appear as dark gaps when atoms absorb light at those same wavelengths. Spectral lines act as a unique fingerprint that allows astronomers to identify the chemical composition of distant stars and galaxies.

mechanics · intro

Speed

The rate at which an object covers distance, calculated by dividing distance traveled by the time taken, and expressed as a scalar quantity with magnitude but no specified direction. Speed differs from velocity, which is the corresponding vector quantity that also specifies the direction of motion. Average speed considers total distance over total time, while instantaneous speed describes how fast an object is moving at a specific moment.

relativity · intro

Speed of Light

The speed at which light and all other massless particles or fields travel through a vacuum, exactly 299,792,458 meters per second, a fundamental constant of nature denoted by the letter c. According to special relativity, this speed represents an absolute cosmic speed limit; no object with mass can be accelerated to reach it, since doing so would require infinite energy. The speed of light also appears in Einstein's mass-energy equivalence relation.

waves · intro

Speed of Sound

The speed at which sound waves travel through a given medium, dependent primarily on the medium's stiffness and density; sound generally travels fastest through solids, slower through liquids, and slowest through gases. In dry air at room temperature, sound travels at approximately 343 meters per second, though this speed increases with air temperature. The speed of sound serves as the reference value for calculating an object's Mach number.

quantum · intermediate

Spin (Quantum)

An intrinsic form of angular momentum carried by elementary particles, distinct from any actual physical spinning motion, that is quantized to take only specific discrete values. Electrons and other fermions have half-integer spin, while force-carrying bosons like the photon have integer spin, a distinction that determines fundamentally different statistical behavior between the two particle categories. Spin also underlies phenomena such as magnetic resonance imaging and the intrinsic magnetism of electrons.

particle · intro

Standard Model

The current, well-tested theoretical framework in particle physics describing all known elementary particles, including quarks, leptons, and force-carrying bosons, along with three of the four fundamental forces: electromagnetism, the weak force, and the strong force. Developed throughout the latter half of the twentieth century, the Standard Model has been confirmed by decades of experimental results, including the 2012 discovery of the Higgs boson, though it notably does not incorporate gravity or explain dark matter.

waves · intro

Standing Wave

A wave pattern that appears stationary in space, formed when two waves of the same frequency and amplitude travel in opposite directions and interfere with each other, typically arising from a wave reflecting back along the same path it came from. Standing waves have fixed points called nodes, where the amplitude is always zero, and antinodes, where the amplitude oscillates at its maximum. Standing waves on a guitar string determine the specific musical notes it can produce.

astro · intro

Star

A massive, self-luminous sphere of plasma held together by its own gravity, generating energy primarily through nuclear fusion reactions in its core, which fuse lighter elements into heavier ones and release the light and heat that make stars shine. Stars form from collapsing clouds of gas and dust and evolve through predictable stages over millions to billions of years, ultimately ending their lives as white dwarfs, neutron stars, or black holes, depending on their initial mass.

em · intro

Static Electricity

The buildup of electric charge on the surface of an object, typically caused by friction between two materials that transfers electrons from one surface to the other, leaving one object with excess positive charge and the other with excess negative charge. This accumulated charge remains stationary until it finds a path to discharge, sometimes producing a visible spark, as commonly experienced after walking across a carpet and then touching a metal doorknob.

thermo · intermediate

Statistical Mechanics

The branch of physics that uses probability and statistics to connect the microscopic behavior of a system's individual particles to its observable macroscopic properties, such as temperature, pressure, and entropy. Rather than tracking every individual particle in a gas containing billions of billions of molecules, statistical mechanics uses averages and probability distributions to derive reliable predictions about the system's overall behavior, providing the theoretical foundation for classical thermodynamics.

thermo · intermediate

Stefan-Boltzmann Law

A law of thermodynamics stating that the total energy radiated per unit surface area by a blackbody is proportional to the fourth power of its absolute temperature. This steep dependence on temperature means that even a modest increase in an object's temperature causes a dramatic increase in the amount of energy it radiates, which is why hotter stars are vastly more luminous per unit surface area than cooler ones.

quantum · intermediate

Stimulated Emission

A process in which a photon interacting with an atom or molecule already in an excited energy state triggers the emission of a second photon that is identical in wavelength, phase, and direction to the first. This process, first described theoretically by Einstein in 1917, is the fundamental physical mechanism that makes lasers possible, since it allows a single photon to trigger a cascading amplification of identical, coherent photons.

particle · intro

Strong Force

Also called: strong nuclear force

One of the four fundamental forces of nature, responsible for binding quarks together to form protons, neutrons, and other hadrons, and for holding protons and neutrons together within an atomic nucleus despite the electrical repulsion between positively charged protons. The strong force is the most powerful of the fundamental forces at short range but decreases rapidly beyond the scale of an atomic nucleus, and it is mediated by particles called gluons.

astro · intro

Sun

The star at the center of our solar system, a massive sphere of mostly hydrogen and helium plasma that generates energy through nuclear fusion in its core, converting hydrogen into helium and releasing the light and heat that sustain life on Earth. Classified as a main-sequence star of average size and temperature compared to other stars, the Sun accounts for over ninety-nine percent of the total mass in the solar system.

cosmology · intermediate

Sunyaev-Zeldovich Effect

A distortion in the observed spectrum of the cosmic microwave background caused when its photons pass through the hot, ionized gas found within massive galaxy clusters and gain a small amount of energy through interactions with high-energy electrons. Because this effect depends on the gas's properties rather than on distance in the usual sense, it provides astronomers with a distance-independent method for detecting massive galaxy clusters throughout the observable universe.

quantum · intermediate

Superconductivity

A phenomenon in which certain materials, when cooled below a characteristic critical temperature, lose all electrical resistance entirely and can conduct electric current indefinitely without any energy loss. Superconductors also expel magnetic fields from their interior, an effect called the Meissner effect, allowing them to levitate above magnets. Superconductivity has practical applications in powerful electromagnets used in MRI machines and particle accelerators, though most known superconductors require extremely low temperatures to function.

astro · intro

Supernova

A powerful and extremely luminous stellar explosion, occurring either when a massive star exhausts its nuclear fuel and its core collapses catastrophically, or when a white dwarf in a binary system accumulates too much mass and undergoes a runaway nuclear reaction. A supernova can briefly outshine an entire galaxy and scatters heavy elements forged within the star into surrounding space, seeding future generations of stars and planets with the material needed to form them.

waves · intro

Superposition

The principle stating that when two or more waves overlap in the same region of space, the resulting displacement at any point is simply the sum of the displacements each individual wave would have produced on its own. Superposition underlies the phenomena of constructive and destructive interference. In quantum mechanics, superposition takes on a deeper meaning, describing how a quantum system can exist in a combination of multiple possible states simultaneously until measured.

mechanics · intro

Surface Tension

The tendency of the surface of a liquid to behave like a stretched elastic membrane, arising because molecules at the surface experience a net inward pull from neighboring molecules, unlike molecules within the bulk liquid, which are pulled equally in all directions. Surface tension allows small insects to walk on water and causes water droplets to form roughly spherical shapes, since a sphere minimizes surface area for a given volume.

math-methods · intermediate

Symmetry (Physics)

A property of a physical system that remains unchanged, or invariant, under a specific transformation, such as rotation, reflection, or translation in space or time. Symmetries play a profound role in physics, since, according to Noether's theorem, every continuous symmetry of a physical system corresponds to a conserved quantity; for example, symmetry under time translation corresponds to conservation of energy, and symmetry under spatial translation corresponds to conservation of momentum.

particle · intermediate

Tau Particle

A subatomic particle similar to the electron and muon but far more massive, roughly 3,477 times heavier than the electron, classified as a lepton and carrying the same negative electric charge. Discovered in 1975, the tau is unstable and decays extremely quickly, with a lifetime of about 2.9 times ten to the minus thirteen seconds, making it the heaviest of the three known charged leptons.

thermo · intro

Temperature

A measure of the average kinetic energy of the particles within a substance, determining the direction of spontaneous heat flow between objects in contact; heat flows from higher to lower temperature until thermal equilibrium is reached. Temperature is measured using scales such as Celsius, Fahrenheit, and the absolute Kelvin scale, and it is a statistical property that only makes clear sense for systems containing many particles.

mechanics · intro

Tension (Force)

A pulling force transmitted through a flexible object such as a rope, cable, or string when it is pulled taut from both ends, directed along the length of the object and away from whatever it is attached to. Tension is a reactive force; it arises only in response to an applied pulling force and adjusts in magnitude to satisfy the physical constraints of the system, such as keeping an inextensible rope from stretching.

mechanics · intro

Terminal Velocity

The constant maximum speed reached by an object falling through a fluid, such as air, when the resistive drag force acting upward exactly balances the downward pull of gravity, resulting in zero net acceleration. Once terminal velocity is reached, the object continues falling at that constant speed rather than continuing to accelerate. Terminal velocity depends on an object's mass, cross-sectional area, shape, and the density of the fluid it is falling through.

em · intro

Tesla (Unit)

The SI unit of magnetic field strength, or magnetic flux density, named after inventor and engineer Nikola Tesla. One tesla represents a fairly strong magnetic field; Earth's own magnetic field at its surface is only about 25 to 65 microteslas, while a typical refrigerator magnet produces roughly 0.001 tesla, and powerful MRI machines and particle accelerator magnets can reach several tesla.

thermo · intermediate

Thermal Conductivity

A material property describing how effectively it conducts heat, measured as the rate of heat transfer through a unit thickness of the material per unit temperature difference. Metals generally have high thermal conductivity due to their free-moving electrons, which efficiently transport thermal energy, while materials like foam, wood, and air have low thermal conductivity and are used as thermal insulators.

thermo · intro

Thermodynamics

The branch of physics concerned with heat, temperature, energy, and their relationships to work and the physical properties of matter, particularly in relation to macroscopic systems in or near equilibrium. Thermodynamics is built around a small number of foundational laws describing the conservation of energy, the tendency of entropy to increase, and the impossibility of reaching absolute zero, and it underlies the operation of engines, refrigerators, and countless natural processes.

mechanics · intermediate

Tidal Force

A differential gravitational force that arises because different parts of an extended object experience slightly different gravitational pull from a nearby massive body, stretching the object along the line toward that body and compressing it perpendicular to that line. Tidal forces from the Moon and Sun cause Earth's ocean tides, and extreme tidal forces near a black hole can stretch and ultimately tear apart an object falling toward it, an effect sometimes called spaghettification.

relativity · intermediate

Time Dilation

A consequence of special and general relativity in which time is measured to pass at different rates for observers in relative motion or experiencing different gravitational strengths. According to special relativity, a clock moving relative to an observer appears to run slower than a stationary one from that observer's perspective, while general relativity predicts that clocks closer to a massive object, in a stronger gravitational field, run slower than clocks farther away. Both effects have been confirmed experimentally.

mechanics · intro

Torque

A measure of the rotational effect of a force applied to an object, calculated as the force multiplied by the perpendicular distance from the axis of rotation to the line along which the force acts, commonly called the lever arm. Torque causes angular acceleration in the same way that ordinary force causes linear acceleration, and it explains why applying force farther from a hinge, such as at the handle of a door, makes it easier to open than pushing near the hinge itself.

optics · intermediate

Total Internal Reflection

An optical phenomenon in which a light ray traveling within a denser medium strikes the boundary with a less dense medium at an angle greater than a specific critical angle, causing all of the light to reflect back into the denser medium rather than refracting through the boundary. Total internal reflection is the physical principle that allows optical fibers to guide light over long distances with minimal loss, since the light continually reflects along the fiber's internal walls.

em · intro

Transformer

An electrical device that transfers electrical energy between two or more circuits through electromagnetic induction, typically used to step voltage up or down while maintaining nearly constant power. A transformer consists of two coils of wire wound around a shared magnetic core; a changing current in the primary coil induces a changing magnetic field, which in turn induces a voltage in the secondary coil, with the ratio of voltages set by the ratio of the coils' turns.

quantum · intro

Transistor

A semiconductor device used to amplify or switch electrical signals, consisting of three terminals through which a small current or voltage at one terminal controls a much larger current flow between the other two. Invented in 1947, the transistor replaced bulkier and less reliable vacuum tubes and became the fundamental building block of modern electronics, with billions of microscopic transistors now integrated into a single computer processor chip.

waves · intro

Transverse Wave

A wave in which the oscillation of the medium, or the oscillating field for electromagnetic waves, occurs perpendicular to the direction the wave travels. Light waves and waves traveling along a plucked string or shaken rope are common examples of transverse waves. This contrasts with longitudinal waves, such as sound, in which the oscillation occurs parallel to the direction of wave travel.

nuclear · intro

Tritium

A radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons, making it roughly three times as massive as ordinary hydrogen. Tritium decays through beta decay with a half-life of about 12.3 years and is used as fuel in certain nuclear fusion reactions, in self-powered lighting devices, and as a tracer in scientific research, though its radioactivity requires careful handling.

mechanics · intermediate

Turbulence

A chaotic, irregular pattern of fluid flow characterized by swirling eddies and rapid, unpredictable variations in velocity and pressure, occurring when a fluid's inertial forces dominate over its viscous forces, typically at high speeds or around obstacles. Turbulence stands in contrast to smooth, orderly laminar flow, and while it increases drag and mixing, it remains one of the most difficult phenomena in classical physics to predict precisely, despite being well described statistically.

cosmology · intro

Type Ia Supernova

A specific class of stellar explosion that occurs when a white dwarf star in a binary system accumulates enough mass, typically by pulling material from a companion star, to trigger a runaway thermonuclear explosion that completely destroys the white dwarf. Because these explosions consistently reach a similar peak intrinsic brightness, Type Ia supernovae serve as reliable standard candles for measuring cosmic distances, and their study in the late 1990s led to the discovery of the accelerating expansion of the universe attributed to dark energy.

em · intro

Ultraviolet Radiation

Electromagnetic radiation with wavelengths shorter than visible light but longer than X-rays, carrying enough energy to cause chemical reactions and biological effects, including sunburn and, with excessive exposure, an increased risk of skin cancer. The Sun emits significant ultraviolet radiation, most of which is absorbed by Earth's ozone layer before reaching the surface. Ultraviolet light is also used in sterilization, fluorescent lighting, and certain types of astronomical observation.

cosmology · intro

Universe

The entirety of space, time, matter, and energy that exists, including all galaxies, stars, planets, and the vast stretches of space between them, along with the fundamental physical laws that govern their behavior. The observable universe, the portion accessible to us given the finite speed of light and the age of cosmic expansion, contains an estimated hundreds of billions of galaxies, though the full extent of the universe beyond what is observable remains unknown.

quantum · intro

Vacuum

A region of space that is largely devoid of matter, containing far fewer particles than the surrounding environment, though a perfect vacuum with absolutely no particles is never achieved in practice, even in the depths of interstellar space. In quantum field theory, even an idealized vacuum is not entirely empty, since quantum fields still exhibit fluctuations, giving rise to phenomena such as the theoretical prediction of virtual particles and zero-point energy.

quantum · intro

Valence Electron

An electron occupying the outermost energy shell of an atom, which participates directly in forming chemical bonds with other atoms. The number of valence electrons an atom has largely determines its chemical properties and its position within a group on the periodic table. In metals, valence electrons are often loosely bound and free to move throughout the material, which is why metals conduct electricity well.

thermo · intermediate

Van der Waals Force

A relatively weak attractive force that exists between neutral atoms or molecules, arising from temporary fluctuations in electron distribution that create brief, shifting regions of positive and negative charge, which then induce corresponding shifts in neighboring molecules. Although much weaker than chemical bonds, van der Waals forces are responsible for phenomena such as the ability of geckos to cling to smooth surfaces and the condensation of gases like noble gases into liquids at low temperatures.