Standard physics glossary

em · intro

Radio Wave

The lowest-energy, longest-wavelength form of electromagnetic radiation, with wavelengths ranging from about a millimeter to hundreds of kilometers. Radio waves are widely used for communication, including broadcast radio, television, mobile phones, and Wi-Fi, because they travel long distances, pass through many materials, and can be generated and detected relatively easily using antennas. Astronomers also use radio telescopes to study emissions from distant galaxies, pulsars, and the cosmic microwave background.

nuclear · intro

Radioactive Decay

The spontaneous transformation of an unstable atomic nucleus into a different, more stable configuration, releasing energy and particles in the process, such as alpha particles, beta particles, or gamma rays. Each radioactive isotope decays at a statistically predictable rate, characterized by its half-life, though the exact moment any individual nucleus decays is fundamentally random and cannot be predicted. Radioactive decay is used in applications ranging from power generation to medical treatment and dating ancient materials.

thermo · intermediate

Random Walk

A mathematical model describing a path made up of a series of random steps, in which each step's direction is independent of the previous ones, used to describe processes such as the diffusion of particles, stock price fluctuations, and the motion of a molecule undergoing Brownian motion. Over many steps, a random walk tends to spread out gradually from its starting point in a statistically predictable way, even though each individual step is unpredictable.

astro · intro

Redshift

A shift of light or other electromagnetic radiation toward longer, redder wavelengths, caused by the relative motion of a source moving away from an observer, by the expansion of space itself over cosmic distances, or by light climbing out of a strong gravitational field. Cosmological redshift, observed in the light from distant galaxies, provides key evidence for the ongoing expansion of the universe, with more distant galaxies generally showing greater redshift.

optics · intro

Reflection

The bouncing back of a wave, such as light or sound, when it encounters a boundary between two different media instead of passing through. For light hitting a smooth, flat surface, the angle at which the wave reflects equals the angle at which it strikes the surface, a relationship known as the law of reflection. Mirrors rely on highly efficient, regular reflection, while rough surfaces produce diffuse reflection, scattering light in many directions.

optics · intro

Refraction

The bending of a wave, most commonly light, as it passes from one medium into another with a different density or optical property, caused by a change in the wave's speed at the boundary. The amount of bending depends on the angle of incidence and the relative refractive indices of the two media, as described by Snell's law. Refraction is responsible for phenomena such as a straw appearing bent in a glass of water and the focusing action of lenses.

optics · intro

Refractive Index

A dimensionless number describing how much a material slows down and bends light compared to its speed in a vacuum, calculated as the speed of light in a vacuum divided by the speed of light within the material. A higher refractive index means light travels more slowly through the material and bends more sharply when entering or leaving it. Refractive index generally varies slightly with wavelength, a phenomenon responsible for the dispersion of light into colors by a prism.

cosmology · intermediate

Reionization

A major transition in the universe's history during which the neutral hydrogen gas that filled intergalactic space became ionized once again, as ultraviolet radiation from the first generations of stars and galaxies stripped electrons from hydrogen atoms throughout the cosmos. This process is believed to have occurred gradually, completing roughly one billion years after the standard hot early expansion phase, and it fundamentally changed how light could travel through intergalactic space.

em · intro

Resistance (Electrical)

A measure of how strongly a material opposes the flow of electric current through it, measured in ohms, arising from collisions between moving charge carriers and the atoms of the material. Resistance depends on a material's inherent resistivity, as well as its length, cross-sectional area, and temperature. According to Ohm's law, resistance relates the voltage across a component to the current flowing through it.

waves · intermediate

Resonance

A phenomenon in which a system experiences dramatically increased amplitude of oscillation when it is driven by an external force at, or very near, its natural frequency. Resonance explains why a wine glass can shatter when exposed to a sound at exactly its resonant frequency, and why soldiers historically break step when crossing bridges, to avoid inadvertently driving the structure at a dangerous resonant frequency. It is exploited deliberately in musical instruments and radio tuning circuits.

relativity · intermediate

Rest Mass

Also called: invariant mass

The mass of an object as measured in a reference frame where the object is at rest, also called invariant mass, since it remains the same value regardless of the observer's relative motion, unlike an object's total relativistic energy, which does depend on relative velocity. Rest mass is the quantity that appears in Einstein's mass-energy equivalence relation for a stationary object, and it represents the intrinsic mass a particle has independent of its motion.

nuclear · intro

Rutherford Scattering

A landmark experiment conducted in 1909 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford, in which alpha particles were fired at a thin sheet of gold foil. Most particles passed through with little deflection, but a small fraction bounced back at large angles, an unexpected result that led Rutherford to conclude that atoms contain a small, dense, positively charged nucleus, replacing the earlier plum pudding model of atomic structure.