Physics

Interpretations of quantum mechanics Survey of the main ways physicists and philosophers understand quantum theory—its competing pictures of the wavefunction, measurement, reality, and probability.

Inverse-square law: definition, mathematics, examples and limitations Inverse-square law Physical principle stating that intensity or force from a point-like source in three dimensions falls in proportion to 1/r^2; applies to gravity, electrostatics, light, sound and o…

Ionizing radiation: nature, types, sources, effects and uses Ionizing radiation Ionizing radiation consists of energetic particles or photons that remove electrons from atoms or molecules. It occurs naturally and from human activities, with medical, industrial…

Iron filings: properties, magnetic behavior, uses and safety Iron filings Small particles of iron produced by machining; used to visualize magnetic fields, recycled for metalwork, and employed in powder metallurgy. Also notable for ferromagnetic behavior…

Island of Stability (nuclear physics) Island of stability Hypothesized region of relatively long-lived superheavy nuclei predicted by the nuclear shell model, its candidate magic numbers, experimental searches, and potential significance.

James Clerk Maxwell: life and scientific contributions James Clerk Maxwell Scottish scientist James Clerk Maxwell (1831–1879) unified electricity, magnetism and light, developed statistical ideas in physics, and made lasting advances in color photography…

Jerk (physics): the rate of change of acceleration Jerk Jerk is the time rate of change of acceleration, a vector quantity measured in m/s³. It matters in ride comfort, control systems and mechanical design, and is related to concepts s…

Joule (unit of energy) Joule The joule is the SI derived unit of energy, work and heat. Defined as one newton-metre, it links mechanical, electrical and thermal quantities and is widely used across science and…

Joule per mole (J·mol⁻¹): unit of energy per amount of substance Joule per mole The joule per mole (J/mol) expresses energy normalized to amount of substance. Common in chemistry and thermodynamics for enthalpy, free energy, bond energies and activation barrie…

Joule's laws: electrical heating and the thermodynamics of ideal gases Joule's laws Two laws formulated by James Prescott Joule: the electrical heating law (Q = I²Rt) and the thermodynamic result that an ideal gas's internal energy depends only on temperature; use…

Kaon (K meson): properties, types, and significance Kaon Kaons are mesons made from a strange quark and an up or down quark. This article covers their quark composition, charge states, neutral-kaon mixing, CP violation, history, and impo…

Kelvin (unit of thermodynamic temperature) Kelvin Kelvin is the SI base unit of thermodynamic temperature. It measures absolute temperature from absolute zero, is named for William Thomson (Lord Kelvin), and is widely used in scie…

Kennelly–Heaviside layer (E region of the ionosphere) Kennelly-Heaviside layer The Kennelly–Heaviside layer, or E region, is a layer of the ionosphere around 90–150 km altitude that ionizes under solar radiation and affects radio propagation, navigation, and…

Kennelly–Heaviside layer (E region of the ionosphere) Kennelly–Heaviside layer A layer of ionized atmosphere about 90–150 km above Earth that affects radio wave propagation. Historically important for long-range AM and shortwave communication and known for da…

Kilogram: SI base unit of mass Kilogram The kilogram is the International System (SI) base unit of mass. Defined in 2019 by fixing the Planck constant, it underlies science, engineering and commerce and is realized by pr…

Kilogram per cubic metre (kg/m³) Kilogram per cubic metre SI unit of density representing mass in kilograms contained in one cubic metre of volume; used in science and engineering to express material and fluid density.

Kilometres per hour (km/h): definition, use, and conversions Kilometres per hour Kilometres per hour (km/h) is a common unit for speed and everyday motion. This article explains its definition, notation, conversions, typical uses, history and distinctions from…

Kinematics: the geometry and description of motion Kinematics Kinematics studies how points, bodies and systems move, describing paths, velocity and acceleration without considering forces. It underpins robotics, biomechanics, engineering and…

Kinetic energy (energy of motion) Kinetic energy Kinetic energy is the energy an object possesses due to motion. This article explains its classical formulas, forms, history, applications, and important distinctions including fra…

Kinetic Theory of Gases (Kinetic–Molecular Theory) Kinetic theory A concise overview of kinetic theory: its basic assumptions, how molecular motion explains pressure and temperature, historical development, applications, and limitations.

Kinetics (mechanics) Kinetics Kinetics is the branch of classical mechanics that relates the motion of bodies to the forces and moments that produce it, distinct from kinematics and also different from chemical…

Laminar flow Laminar flow is a smooth, orderly type of fluid motion in which layers slide past one another with minimal mixing; common at low velocities, small scales or high viscosity and gove…

Large Hadron Collider (LHC) Large Hadron Collider The Large Hadron Collider is a 27 km circular particle accelerator at CERN beneath the Swiss–French border. It accelerates hadrons to near light speed to study fundamental particle…

Laser: principles, types, history and applications Laser A laser is a device that produces intense, coherent, directional light by stimulated emission. This article explains how lasers work, common types, historical development, uses, an…

Latent heat (energy involved in phase changes) Latent heat Latent heat is the energy absorbed or released when a substance changes phase without a temperature change. It explains melting, boiling, condensation, and underpins many technolog…

Laws of Conservation Fundamental principles that certain physical quantities remain constant in isolated systems — including energy, momentum, charge — rooted in symmetries and expressed as continuity…

LC circuit: principles, behavior, and common applications LC circuit An LC circuit pairs an inductor (L) and a capacitor (C) to form a resonant electrical network used in tuning, filtering, and oscillators; ideal LC is lossless and exchanges energy…

Length (measurement and concepts) Length Length is a measure of distance between two points along one dimension. This article explains definitions, units, methods of measurement, historical development, common tools, and…

Lens: optical elements, types, and applications Lens Overview of lenses: optical principles, common shapes and materials, manufacturing and coatings, principal uses from eyeglasses to telescopes, and related meanings like gravitation…

Lens (optics): overview, types, history and uses Lens (optics) A lens is a transparent optical element that refracts light to form images. This article explains how lenses work, common shapes and types, focal length and power, history, and key…

Lenz's Law: Electromagnetic induction and opposition to changing magnetic flux Lenz's law Lenz's law states that an induced electromotive force produces a current whose magnetic field opposes the change in magnetic flux. It gives the negative sign in Faraday's law and e…

Leptons — elementary fermions not subject to the strong interaction Lepton Leptons are spin-1/2 elementary particles of the Standard Model that do not feel the strong nuclear force. There are six flavors: three charged (electron, muon, tau) and three neut…

Lever (simple machine): principles, types, uses, and history Lever A lever is a rigid bar that pivots on a fulcrum to move or measure loads. This article explains parts, classes (first, second, third), mechanical advantage, historical context, and…

Leyden jar — early glass capacitor for storing static electricity Leyden jar An 18th‑century device for storing static electricity. The Leyden jar was the first practical capacitor, widely used in early experiments to demonstrate charge storage, sparks and…

Lift (force): Principles, mechanisms and applications Lift (force) Lift is the net force on a body perpendicular to the flow of a fluid. This article explains how lift arises, common misconceptions, practical applications, limits such as stall, an…

Light: nature, spectrum, behavior, sources and uses Light Overview of visible light as electromagnetic radiation: its dual nature, spectrum and colors, physical properties, common behaviors, principal sources like the Sun, and practical a…

Light beam A light beam is a concentrated projection of electromagnetic energy from a source. This article explains how beams form, their physical properties, common devices that produce them…

Light clock (special relativity thought experiment) Light clock A pedagogical thought experiment using a light pulse between mirrors to illustrate invariant light speed and time dilation; includes construction, derivation, examples, assumptions…

Light-second A light-second is the distance light travels in vacuum in one second — exactly 299,792,458 metres — used to express distances and signal delays in astronomy, telecommunications and…

Lightning (Atmospheric Electrical Discharge) Lightning Lightning is a rapid, high-voltage electrical discharge in the atmosphere that forms from charge separation in clouds, producing bright flashes and thunder; it occurs within clouds…

Lise Meitner: Pioneer of Nuclear Fission and 20th‑Century Physics Lise Meitner Austrian‑Swedish physicist Lise Meitner (1878–1968) contributed to the discovery and interpretation of nuclear fission, advanced radioactive research, and left a complex scientific…

Notable Physicists: Key Figures in Classical, Quantum, and Modern Physics List of physicists Directory of influential physicists across classical, quantum, and modern physics, offering concise biographies, key contributions, and areas of specialization for students and res…

List of wave topics: overview, categories, and key concepts List of wave topics An organized guide to topics related to waves across physics, mathematics, engineering and Earth sciences, with major categories, phenomena, history, applications and distinctions.

Locating in Physics: From Coordinates to Quantum Limits Locating (physics) How scientists define and determine where something is: coordinate systems, measurement methods, practical techniques, and the conceptual limits imposed by waves and quantum mechan…

Longitudinal wave A longitudinal wave is a mechanical disturbance in which particles of the medium oscillate parallel to the wave’s direction, producing compressions and rarefactions; sound and seis…

Lorentz contraction (length contraction) Lorentz contraction Lorentz contraction is the apparent shortening of an object’s length measured along its direction of motion, predicted by special relativity and arising from differences in simulta…

Lorentz factor (γ) — the relativistic scaling of time, length and mass Lorentz factor The Lorentz factor γ = 1/√(1−(v/c)²) quantifies how time, distance and energy scale at speeds near light. It appears in Lorentz transformations, time dilation, length contraction a…

Lorentz force The Lorentz force law describes the electromagnetic force on a charged particle moving in electric and magnetic fields: F = q(E + v × B). It explains particle motion, devices, and…