Physics Glossary & Definitions

phy-1109
PHY-1109 Physics

I. Oscillations and Mechanical Waves

Term/Concept	Definition and Notes	Year(s) of Appearance
Simple harmonic motion (SHM)	Motion where the restoring force ( $F$ ) is proportional to the negative of the displacement ( $F \propto - x$ ). The differential equation for SHM is $\frac{d ^{2} y}{d t ^{2}} + ω^{2} y = 0$ , where $ω = k / m$ .	2016, 2017, 2018, 2021, 2022, 2023, 2024
Damped vibrations	Oscillations where resistive forces (damping, proportional to velocity, $F \propto - b v$ ) act alongside the restoring force. The resultant displacement decreases over time, typically exponentially, such as in the most significant under-damped case.	2017, 2020, 2018, 2019, 2021
Forced vibrations	Vibrations resulting from the application of an external periodic force, typically represented by $F = F_{0} sin (ω t)$ . The complete solution for forced vibration includes both damped and forced terms.	2017, 2020
Resonance	The condition in forced vibration where the amplitude ( $A$ ) of oscillation reaches its maximum value. This occurs when the driving frequency ( $p$ ) satisfies the condition $p^{2} = ω^{2} - 2 λ^{2}$ , where $ω$ is the natural frequency and $λ$ is the damping constant.	2017, 2020
Bandwidth of resonance	The bandwidth of resonance is the range of frequencies around the resonant frequency over which a system (like an RLC circuit or mechanical oscillator) responds significantly.	2019
Quality factor ( $Q$ )	Defined as $Q = 2 π \times \frac{Energy stored per cycle}{Energy loss per cycle}$ . When damping ( $λ$ ) is very small, the Quality factor is approximately $Q = \frac{ω}{2 λ}$ .	2024
Wave velocity (Phase velocity, $w$ )	The velocity of a point of constant phase on a single monochromatic wave component. Mathematically, it is given by $w = ν λ$ or $w = ω / k$ . For De Broglie waves, the phase velocity is $w = c^{2} / v$ , which is greater than the speed of light ( $c$ ).	2018, 2021, 2022, 2023
Stationary waves	A resultant wave formed by the interference between two progressive waves of equal amplitude and frequency traveling in opposite directions (often due to reflection, e.g., from a fixed or open end).	2017, 2021, 2019
Nodes	Positions in a stationary wave where the displacement and amplitude are permanently zero. For fixed-end reflection, the positions are $x = m (λ /2)$ , where $m = 0, 1, 2, \dots$ .	2024(becm)
Anti-nodes	Positions in a stationary wave where the displacement amplitude is maximum. For fixed-end reflection, these points are found at $x = (2 m + 1) (λ /4)$ .	2024(becm)
Group velocity ( $v_{g}$ )	The velocity of the envelope formed by a superposition of waves, known as the wave packet. It represents the physical speed at which the particle, energy, or information travels. For De Broglie waves, the group velocity is equal to the particle velocity ( $v_{g} = v$ ). Mathematically, $u = d ω / d k$ .	2023, 2021, 2022
Beats	Phenomena produced by the superposition of two waves (or sound notes) having slightly different frequencies. The number of beats produced per second is equal to the difference in frequency of the two sources ( $n_{1} - n_{2}$ ).	2018, 2022, 2024(becm), 2024
Bel (B)	A logarithmic unit for measuring sound intensity level, defined by $L = lo g_{10} (I / I_{0})$ Bel (or 10 times this value for the more commonly used unit, the decibel, dB).	2016, 2024(becm)
Phone	A phon is a unit of loudness level, based on human hearing perception.	2016, 2024(becm)
De Broglie Wavelength ( $λ$ )	The wavelength associated with a moving material particle, generalized from the photon momentum relation. Defined by $λ = \frac{h}{p} = \frac{h}{mv}$ .	External
Wave Number ( $k$ )	The quantity $k = \frac{2 π}{λ}$ . It is equal to the number of radians corresponding to a wave train 1 m long. The unit is radian per meter.	External
Reverberation	The persistence of audible sound even after the sound source has stopped.	External
Reverberation Time ( $T$ )	The time required for the sound intensity in a room to decay by a factor of $1 0^{6}$ (or $60$ dB) after the source stops. Calculated using Sabine’s formula: $T = \frac{0.16 V}{A}$ (in meters).	External

II. Optics

Term/Concept	Definition and Notes	Year(s) of Appearance
Interference of light	The phenomenon wherein two wave trains act simultaneously on any particle in a medium, causing the displacement of the particle at any instant to be due to the superposition of all the wave trains.	2016, 2023, 2017
Coherent sources of light	Two sources that emit light waves of the same frequency and nearly the same amplitude, and are always in phase (or maintain a constant phase difference) with each other. Coherent sources must emit radiation of the same color (wavelength).	2017, 2019, 2024
Newton’s ring	Alternating bright and dark concentric rings formed due to interference phenomena when a plano-convex lens of long focal length is placed on a plane glass plate, enclosing a thin, variable-thickness air film.	2019, 2023
Diffraction of light	The spreading of a beam of light into the region of the geometrical shadow when the light passes through a small opening (like a narrow slit) or around the edges of an opaque obstacle.	2023
Resolving power of a grating	Quantified using the Rayleigh criterion: two wavelengths in a line spectrum are considered resolved if the maximum in the diffraction pattern from the first wavelength coincides with the minimum in the diffraction pattern from the second wavelength.	2022
Polarization of light	A phenomenon that has helped to establish definitively that light waves are transverse waves (as experiments on interference and diffraction alone do not distinguish wave type). Polarization of light by reflection from surfaces like glass was discovered by Malus in 1808.	2016, 2018
Polarizer	An optical device used specifically for producing plane polarized light. The Nicol prism is an example of a polarizer.	2022
Analyzer	An optical device used for the detection of plane polarized light. The Nicol prism can also function as an analyzer.	2022
Nicol prism	An optical device, invented by William Nicol in 1828, constructed from a calcite crystal and used for producing and analyzing plane polarized light.	2023
Malus law	The law states that the intensity of the polarized light ( $E_{1}$ ) transmitted through the analyzer varies as the square of the cosine of the angle ( $θ$ ) between the plane of transmission of the analyzer and the plane of the polarizer, derived as $E_{1} = E cos^{2} θ$ (or $E_{1} \propto cos^{2} θ$ ).	2021
Brewster’s law	The law, derived from Brewster’s experiments, proves that the tangent of the angle of polarization ( $i$ ) is numerically equal to the refractive index ( $μ$ ) of the medium ( $μ = tan i$ ). This condition occurs when the reflected and refracted rays are perpendicular to each other.	2021
Specific rotation (of an optically active substance)	Defined as the rotation (of the plane of vibration) produced by a decimeter (10 cm) long column of the liquid containing 1 gm of the active substance in one cc of the solution.	2019, 2023
Equivalent lens	An equivalent lens is a single hypothetical lens that can replace a system of two or more lenses placed in contact, producing the same overall optical effect (same image position, magnification, and power) as the entire lens combination.	2016
Equivalent focal length	Equivalent focal length is the effective focal length of a combination of lenses placed in contact. It is the focal length of the equivalent single lens that has the same total power as the lens system.	2016
Polarizing Angle	The particular angle of incidence (e.g., $57. 5^{\circ}$ for a glass surface) at which light reflected is completely extinguished by placing a tourmaline crystal in its path, yielding polarized light.	External
Double Refraction	The phenomenon discovered by Erasmus Bartholinus in 1669, where a ray of light refracted by a crystal (like calcite) gives two refracted rays.	External
Optical Activity	The phenomenon or property of rotating the plane of vibration of light by certain crystals or substances.	External
Optically Active Substance	A substance that possesses the property of optical activity, meaning it turns the plane of vibration of polarized light.	External
Principal Section of the Crystal	A plane that contains the optic axis and is perpendicular to the opposite faces of the crystal.	External

III. Modern Physics

Term/Concept	Definition and Notes	Year(s) of Appearance
Proper time	Proper time is the time interval measured by a clock that moves together with the object whose motion is being observed.	2021
Photoelectric current	The measured electric current in a photoemission experiment, constituted by the photoelectrons emitted from the metal surface (anode) that reach the cathode. This current is proportional to the light intensity for a given frequency.	2022
Work function ( $φ$ )	The minimum energy ( $h ν_{0}$ ) required for an electron to escape from a particular metal surface. It is the energy equivalent of the threshold frequency ( $ν_{0}$ ).	2022
Threshold frequency ( $ν_{0}$ )	A critical frequency characteristic of a specific metal, below which no photoelectrons are emitted, regardless of the light intensity.	2022
Quanta (Photons)	Small packets in which the energy in light is concentrated, as realized by Einstein in 1905 to explain the photoelectric effect. Each photon of frequency $ν$ carries energy $E = h ν$ . The term photon was coined in 1926.	2023
Space quantization	The phenomenon that causes the total angular momentum vector ( $j$ ) of an electron to take on only discrete orientations relative to an applied external field, leading to the existence of the Magnetic Quantum Number ( $m_{j}$ ).	2024
Radioactivity	Radioactivity is the spontaneous disintegration of unstable atomic nuclei, during which they emit alpha (α), beta (β), or gamma (γ) radiation in order to reach a more stable nuclear state.	2016, 2017, 2023
Nuclear binding energy / Binding energy	Nuclear binding energy is the energy required to completely separate all nucleons of a nucleus into free protons and neutrons. Equivalently, it is the energy released when a nucleus forms from its constituent nucleons a direct measure of nuclear stability.	2020, 2018
Packing fraction (of a nucleon)	Packing fraction is a dimensionless quantity defined as the difference between the actual mass of a nucleus (mass of isotope) and its mass number, divided by the mass number. It indicates how tightly nucleons are packed inside a nucleus and provides insight into nuclear stability.	2018
Heisenberg uncertainty principle	States that it is impossible to simultaneously measure the x-components of position and momentum of a particle with arbitrarily high precision. The product of experimental uncertainties is always larger than or equal to $\frac{1}{2} ℏ$ .	External
Energy-time uncertainty principle	The experimental observation that a quantum state that exists only for a short time cannot have a definite energy.	External
Wave Function ( $Ψ$ )	The quantity whose periodic variations constitute matter waves. The value of $Ψ$ at a point is related to the likelihood (probability) of finding the body there at that time. It has no direct physical significance itself.	External
**Probability Density ($	^2$)**	\Psi
Compton Effect	The scattering event where an x-ray photon strikes an electron (initially at rest) and is scattered away, resulting in the scattered photon having lower energy (longer wavelength) than the incident photon. Energy and momentum are conserved in this event.	External
Compton Wavelength ( $λ_{c}$ )	The quantity defined by $λ_{c} = \frac{h}{m _{0} c}$ , where $m_{0}$ is the rest mass of the scattering particle. This value gives the scale of the wavelength change of the incident photon.	External
Pauli’s Exclusion Principle	Dictates that in a single atom, no two electrons can have the same exact set of values for the four quantum numbers ( $n, l, m_{l}, m_{s}$ or $n, l, j, m_{j}$ ).	External
Principal Quantum Number ( $n$ )	Determines the energy of the electron and the size of the orbit as a whole. Range: $n = 1, 2, 3, \dots, \infty$ .	External
Orbital/Azimuthal Quantum Number ( $l$ )	Determines the shape of the electron orbits and the orbital angular momentum. Range: $l$ varies from $0$ to $(n - 1)$ .	External