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Standard physics glossary
Textbook physics terms. Not a BFUT reinterpretation. 371 terms. Textbook meanings only.
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16 terms. Page 1 of 1.
mechanics · intermediate
Lagrangian Mechanics
A reformulation of classical mechanics, developed by Joseph-Louis Lagrange, that describes motion using a single function called the Lagrangian, typically the difference between a system's kinetic and potential energy, combined with the principle of least action. Rather than analyzing forces directly, Lagrangian mechanics derives equations of motion from energy considerations, which often simplifies problems involving constraints or complex coordinate systems and extends naturally into quantum field theory.
optics · intro
Laser
A device that produces a highly focused, coherent beam of light through a process called stimulated emission, in which excited atoms or molecules are triggered to emit photons of the same wavelength, phase, and direction. The name is an acronym for light amplification by stimulated emission of radiation. Lasers are used across an enormous range of applications, including surgery, telecommunications, industrial cutting, and precision measurement.
thermo · intro
Latent Heat
The amount of heat absorbed or released by a substance during a phase change, such as melting, freezing, boiling, or condensing, without any accompanying change in temperature. For example, converting one kilogram of ice at zero degrees Celsius into water at the same temperature requires a specific, fixed amount of energy known as the latent heat of fusion. Latent heat reflects the energy needed to rearrange molecular bonds rather than to increase molecular motion.
relativity · intermediate
Length Contraction
A consequence of special relativity in which an object's measured length along its direction of motion appears shorter to an observer when the object moves at a significant fraction of the speed of light, relative to that observer. The effect only becomes noticeable at speeds approaching the speed of light and is symmetric: each observer sees the other's objects as contracted. Length contraction is closely related to time dilation.
optics · intro
Lens
A transparent optical component, typically made of glass or plastic, with at least one curved surface, that bends light rays through refraction to converge or diverge them. Convex lenses converge parallel light rays to a focal point and are used to magnify images or correct farsightedness, while concave lenses spread light rays apart and are used to correct nearsightedness. Lenses form the basis of eyeglasses, cameras, microscopes, and telescopes.
em · intermediate
Lenz's Law
A law stating that the direction of an electric current induced by a changing magnetic field always opposes the change that produced it, a consequence of the conservation of energy. Lenz's law explains why moving a magnet toward a coil induces a current that creates a magnetic field opposing the magnet's approach, effectively resisting the motion. It provides the direction that complements the magnitude given by Faraday's law of induction.
particle · intermediate
Lepton
A category of fundamental particle that does not experience the strong nuclear force, including the electron, muon, tau, and their associated neutrinos, along with their antiparticles. Leptons are considered elementary, with no known internal substructure, and are classified as fermions. The electron is the only lepton stable enough to exist as ordinary matter under normal conditions; the others are far more massive and decay quickly.
mechanics · intro
Lift (Aerodynamics)
The upward force generated on an object, such as an aircraft wing, as it moves through a fluid, arising from a pressure difference between the upper and lower surfaces caused by the object's shape and angle relative to the airflow. Lift allows heavier-than-air vehicles to fly by counteracting gravity, and its magnitude depends on the object's speed, shape, surface area, and the density of the surrounding air.
astro · intro
Light-year
A unit of distance, rather than time, equal to the distance light travels in one year through the vacuum of space, approximately 9.46 trillion kilometers. Astronomers use light-years to describe the vast distances between stars and galaxies, since more familiar units like kilometers become unwieldy at cosmic scales. Because light takes time to travel, observing an object a certain number of light-years away also means seeing it as it appeared that many years in the past.
cosmology · intermediate
Lithium Problem
A persistent discrepancy in cosmology between the predicted abundance of lithium-7 from standard Big Bang nucleosynthesis calculations and the actual amount of lithium-7 observed in the atmospheres of old, metal-poor stars, which is consistently measured to be roughly three times lower than predicted. Despite decades of study, no fully satisfying explanation for this discrepancy within the standard cosmological model has been universally accepted, though various astrophysical and nuclear physics explanations have been proposed.
waves · intro
Longitudinal Wave
A wave in which the oscillation of the medium occurs parallel to the direction the wave travels, creating alternating regions of compression and rarefaction. Sound waves in air are the most common example of longitudinal waves, as air molecules vibrate back and forth along the same direction the sound travels. This contrasts with transverse waves, such as light or a wave on a string, where the oscillation is perpendicular to the direction of travel.
em · intermediate
Lorentz Force
The combined electric and magnetic force experienced by a charged particle moving through electric and magnetic fields, calculated as the charge multiplied by the electric field plus the charge times the particle's velocity crossed with the magnetic field. The magnetic component of the Lorentz force acts perpendicular to both the particle's velocity and the magnetic field, causing charged particles to move in curved or helical paths within a magnetic field.
relativity · intermediate
Lorentz Transformation
A set of equations from special relativity that relate the coordinates of space and time as measured by two observers moving at constant velocity relative to each other. Unlike the simple addition of velocities used in classical Newtonian physics, Lorentz transformations account for the fact that the speed of light remains constant for all observers, leading to effects such as time dilation and length contraction at high relative speeds.
astro · intro
Luminosity
The total amount of energy a star or other astronomical object emits per unit time, an intrinsic property independent of the observer's distance from the object, usually expressed in watts or in multiples of the Sun's luminosity. Luminosity depends on both an object's surface temperature and its surface area, so a large, cool star can be more luminous than a small, hot one. It is distinguished from apparent brightness, which also depends on distance.
cosmology · intermediate
Luminosity Distance
A method of expressing the distance to an astronomical object, calculated by comparing the object's known intrinsic luminosity to its observed apparent brightness, accounting for how radiation spreads out and, at cosmological distances, how the expansion of the universe dims and stretches the light along its path. Luminosity distance differs from simple geometric distance for very distant objects, since it must account for the universe's expansion history, making it a valuable tool for testing cosmological models using standard candles such as Type Ia supernovae.
cosmology · intermediate
Lyman-alpha Forest
A dense series of absorption lines observed in the spectra of distant quasars, produced when light passing through numerous clouds of intervening neutral hydrogen gas at different distances and redshifts absorbs specific wavelengths corresponding to the Lyman-alpha electron transition. The resulting forest-like pattern of absorption lines allows astronomers to map the distribution of diffuse intergalactic gas and probe the large-scale structure of the universe at intermediate redshifts.
