Technology

QRpedia: multilingual access to Wikipedia via QR codes QRpedia QRpedia is a web service that uses QR codes and device language settings to deliver Wikipedia articles in the visitor’s preferred language, commonly used by museums, heritage sites…

Quantum computer: principles, architectures, and practical challenges Quantum computer An overview of quantum computers: core concepts (qubits, superposition, entanglement), theoretical models, physical implementations, algorithms, practical limits and potential appl…

Quartz clock: principles, history, accuracy and common uses Quartz clock A quartz clock uses a quartz crystal oscillator to regulate electronic timekeeping. This article explains how it works, its development, accuracy, variants, and common applications…

Queue (abstract data type) A FIFO collection supporting enqueue, dequeue and peek operations. Queues appear in scheduling, buffering, traversal and messaging; variants include circular, blocking and priority…

QuickTime QuickTime is Apple’s multimedia architecture and player for handling digital video, audio, images, animation and interactive panoramas, known for the .mov container and extensible…

Quill pen: history, construction, and modern use Quill A quill is a writing instrument made from a bird feather. This article explains its parts, how it was made and used, its historical role, and why it remains popular in calligraphy…

QWERTY keyboard layout QWERTY QWERTY is the standard Latin-script keyboard layout whose name comes from the first six letters of its top letter row; it originated with 19th-century typewriters and remains domin…

Radar: principles, components, history, types and applications Radar Radar is a system that uses radio waves to detect, locate and track objects. This article explains how radar works, its main parts, historical development, common types, uses, limi…

Radiator (heat exchanger for heating and cooling) Radiator A radiator is a heat exchanger that moves thermal energy between a fluid and surrounding air. Used for building heating, vehicle cooling, and many industrial applications, with var…

Radio Control (RC): Principles, Components, History and Applications Radio control Overview of radio control systems used to operate scale models and unmanned vehicles, explaining components, operation, history, common uses, safety and regulatory considerations.

Radio-frequency identification (RFID): technology, components, uses and issues Radio-frequency identification Comprehensive overview of RFID: how tags and readers work, tag types and construction, frequencies and standards, common applications, deployment considerations, and privacy/securi…

Radiography: principles, equipment, clinical uses and safety Radiography Radiography is the use of X‑rays to produce projection images of internal structures. This article explains how radiography works, common equipment, clinical applications, safety c…

Radioisotope heater unit (RHU) Radioisotope heater unit A radioisotope heater unit is a small device that produces continuous heat from radioactive decay to warm instruments and spacecraft systems in cold or remote environments.

Radioisotope thermoelectric generator (RTG) Radioisotope thermoelectric generator A device that converts heat from radioactive decay into electricity using thermocouples; widely used to power spacecraft and remote systems where solar power is impractical.

Radiometer — instrument for measuring radiant energy Radiometer A radiometer is an instrument that measures radiant power or irradiance of electromagnetic radiation. It includes thermal and photon detectors, with applications from meteorology t…

Railgun: electromagnetic launcher for high-speed projectiles Railgun A railgun uses electromagnetic forces to accelerate solid projectiles without chemical propellants. This article explains how railguns work, their history, design issues, uses, and…

Rainwater tank (rain barrel or water butt) Rainwater tank A rainwater tank is a container that captures roof runoff for later use. Covers types, components, history, common uses, benefits, maintenance and legal considerations.

Random-access memory (RAM): functions, types, history, and uses Random-access memory Overview of random-access memory (RAM): what it is, how it works, main types (DRAM, SRAM, NVRAM), historical development, typical uses, and distinguishing features such as volatili…

Ransomware: malware that restricts access and demands payment Ransomware Ransomware is malicious software that denies access to systems or data, typically by encryption, then demands payment. This article explains how it works, its history, notable inci…

Raspberry Pi: compact single-board computers for learning and projects Raspberry Pi The Raspberry Pi is a low-cost, credit-card sized single-board computer series created to promote computing education and enable hobbyist, industrial and educational projects.

Raster graphics (bitmap images): structure, formats, and uses Raster graphics Raster (bitmap) images represent pictures as grids of pixels. This article describes their structure, common file formats, history, printing workflow, and how they differ from vect…

Raster Image Processor (RIP): function, types, and role in printing Raster image processor A Raster Image Processor (RIP) converts page description languages into printable bitmaps. This article explains how RIPs work, hardware vs software implementations, history, commo…

Razor (personal grooming tool) Razor A razor is a handheld device with one or more blades used to remove hair. This article explains types, history, construction, uses, maintenance, and safety considerations.

RC2: a 64-bit block cipher by Ronald Rivest RC2 RC2 is a symmetric 64-bit block cipher introduced in 1987 by Ronald Rivest. It uses a variable-length key, an 18-round mix/mash structure, and a complex key expansion; historically…

RC4 (Rivest Cipher 4): overview, design, history and security RC4 Comprehensive overview of the RC4 stream cipher: design and operation, historical adoption, known vulnerabilities, mitigation strategies and modern alternatives for secure systems.

RC5 (block cipher): design, parameters, and cryptographic role RC5 RC5 is a parameterized symmetric block cipher designed by Ronald Rivest in 1994, notable for data-dependent rotations, simple implementation, and flexible block/key/round sizes use…

RC6: a parameterized symmetric-key block cipher RC6 RC6 is a symmetric-key block cipher derived from RC5 and submitted to the AES competition. It uses data-dependent rotations, integer multiplication and flexible parameters for bloc…

RCA connector (phono/cinch): analog audio and video plug RCA connector RCA connector, also called phono or cinch, is a common unbalanced analog connector used for consumer audio and video; familiar color coding: yellow (video), white/black (left), red…

ReactOS — an open-source, Windows-compatible operating system ReactOS ReactOS is an open-source project that aims to reimplement the Windows NT architecture to run Windows applications and drivers without using Linux or Microsoft code.

Read-only memory (ROM): overview, types, history, and uses Read-only memory Non-volatile memory that stores firmware and system code. ROM preserves data without power and includes mask ROM, PROM, EPROM, EEPROM and flash; widely used for boot firmware and e…

Real-time computing: concepts, systems, and applications Real-time computing Overview of real-time computing: definitions, properties, history, system types (hard vs soft), scheduling and design concerns, common applications, and notable challenges.

Real-time operating system (RTOS): overview, features, and uses Real-time operating system A real-time operating system (RTOS) is an OS optimized for predictable, low-latency task scheduling in time-sensitive and embedded applications, with distinct design choices and tr…

Real-time strategy (RTS) video games Real-time strategy Real-time strategy (RTS) is a video game genre in which players manage resources, build bases and command units simultaneously, emphasizing tactical decision-making and strategic p…

reCAPTCHA reCAPTCHA is an online challenge service that helps distinguish humans from automated bots using interactive tests (images, checkboxes, audio) and has evolved to support machine le…

Rechargeable battery (secondary cell): overview, types, uses and safety Rechargeable battery A rechargeable battery (secondary cell) stores electrical energy and can be restored by charging. Covers common chemistries, history, typical uses, charging practices, safety and e…

Reconstructor — Ubuntu ISO customization tool Reconstructor Reconstructor is a utility for customizing Ubuntu (and Debian-based) ISO images, enabling package inclusion, branding, and configuration for creating tailored live CDs or installer…

Rectifier: Converting Alternating Current to Direct Current Rectifier Overview of rectifiers: purpose, types (half-wave, full-wave, bridge), components, history, applications, and technical considerations such as ripple, smoothing, and controlled rec…

Red Hat Linux — discontinued community distribution and its legacy Red Hat Linux Red Hat Linux was a widely used, freely distributed Linux distribution produced by Red Hat. Discontinued in 2003, it was succeeded by Red Hat Enterprise Linux (commercial) and Fedo…

Reduced Instruction Set Computer (RISC) Reduced instruction set computer RISC (Reduced Instruction Set Computer) is a CPU design philosophy that emphasizes a small, fast set of simple instructions, large register files, and efficient pipelining to impro…

Reel-to-Reel Tape Recorders: History, Design, Uses, and Legacy Reel-to-reel tape recorders A comprehensive overview of reel-to-reel tape recorders: how they work, their development in the 20th century, technical traits, common uses in studios and homes, upkeep, and cultu…

Referential transparency in programming and mathematics Referential transparency Referential transparency means an expression can be replaced by its value without changing observable behavior. It relies on purity, enables equational reasoning and optimizations,…

Refresh rate (display) Refresh rate Refresh rate is how often a display updates its image each second, measured in hertz (Hz). It affects motion clarity, synchronization with content, and is distinct from frame rate…

Refrigerator: household cooling appliance and its design, history, and uses Refrigerator A refrigerator cools and preserves food and drinks by removing heat. This article explains how refrigerators work, their parts and types, the history from iceboxes to modern models…

Regional lockout: how devices and media enforce geographic restrictions Regional lockout Regional lockout (region coding) restricts use of media or devices to particular geographic areas. It appears in discs, games, printers and streaming; used for market control, lega…

Relational database: structure, theory, and practical uses Relational database An overview of relational databases: definition, core concepts (tables, keys, joins, normalization), history, common uses, advantages, limitations, and how they differ from other d…

Remote control: purpose, components, history and common types Remote control A remote control is a handheld device that issues commands to machines from a distance. This article explains how remotes work, common technologies, components, history, and typica…

Remote keyless system: overview, types, operation and security Remote keyless system Description and explanation of remote keyless systems: how they work, components, common uses in vehicles and buildings, types, security issues and practical mitigations.

Repository (data) A repository is a structured storage location for files and metadata, widely used in version control to track changes. This article explains types, components, operations, history,…