Glossary

absolute zero: the temperature at which an object has zero thermal energy.

absorption lines: dark lines in a spectrum due to the absence of light at particular wavelengths.

absorption spectrum: a spectrum of electromagnetic radiation exhibiting absorption lines.

atoms: the smallest constituent of matter bearing the properties of a chemical element.

binding energy: the amount of energy required to liberate an electron from an atom.

blackbody: an object which is not truly black unless has a temperature of absolute zero, but appears black at room temperature because it is a perfect absorber of radiation, which it emits as a continuous spectrum in infrared.

blackbody radiation: thermal radiation from any dense object, such as a blackbody, which has a continuous spectrum described by Planck’s law.

Bohr model of the atom: an early quantum-mechanical model of an atom which accurately accounts for hydrogen emission spectra.

continuous spectrum: a spectrum of electromagnetic radiation exhibiting no absorption or emission lines.

Coulomb force: the electrical force between two charges.

deuterium: an isotope of hydrogen with one neutron.

Doppler effect: a shift in the length of a wave emitted by a source that is moving relative to an observer.

Doppler shift: the wavelength difference in any spectral feature that results from the Doppler effect.

electromagnetic radiation: radiant energy in the form of oscillating electric and magnetic fields.

electron cloud: a descriptive term used informally to describe where electrons are located around the nucleus.

electrons: subatomic particles with negative charge that exist in orbitals around the nucleus.

element: a chemical substance or atom whose physical properties are principally determined by the number of protons in their nuclei.

emission lines: bright lines in a spectrum due to the presence of light emitted by a source at particular wavelengths.

emission spectrum: a spectrum of electromagnetic radiation exhibiting emission lines.

energy: a physical property which can be transferred between objects or converted into different forms, but which is not created or destroyed.

Fraunhofer lines: another term for absorption lines.

frequency (ν): The rate at which one full wavelength of electromagnetic radiation occurs.

heat: generated by the movement of atoms or molecules within an object.

illuminate: to shine light on something.

ionised: the state of an atom or molecule with nonzero charge due to a difference in the numbers of protons and electrons.

isotope: an atom; the term isotope is used to differentiate between atoms of the same element with different numbers of neutrons.

Kelvin temperature scale: the temperature scale defined by setting its zero-value to absolute zero and a value of 273 to the triple-point of water.

Kirchhoff’s three laws: three laws describing the conditions leading to emission, absorption, and continuous spectra.

luminosity: the total amount of electromagnetic radiation emitted by an object per unit time.

molecules: the smallest particle in a chemical element or compound that bears its chemical properties.

neutrinos: an elementary subatomic particle with no charge.

neutrons: a subatomic particle with no charge and slightly larger mass than a proton.

nuclear fusion: the process by which protons and neutrons combine to form heavier isotopes or atoms.

nucleus: the central part of an atom.

orbital: variously used here in referring to electron orbitals, atomic orbitals, or energy orbitals, all meaning the same thing; the different orbitals of an atom are the quantum values of energy at which an electron can exist.

photons: particles of electromagnetic radiation which are responsible for the photoelectric effect.

Planck’s constant: a fundamental constant of nature, central in quantum mechanics.

Planck’s law: describes the thermal radiation emitted by a blackbody of a definite temperature

power: rate of energy consumed per unit time.

protons: subatomic particles with positive charge and mass slightly less than neutrons.

quantum leap: the quantum transition of an electron in an atom from one orbital to another.

quantum mechanics: the scientific description of matter and its interaction with light.

quantum physics: see quantum mechanics.

quarks: an elementary subatomic particle; quarks combine to form other subatomic particles such as protons and neutrons.

scattered light: light that has been deflected through interactions with subatomic particles.

solar constant: the amount of energy per unit area from the Sun that is received at Earth.

solar neutrino problem: the previously unresolved problem of a discrepancy between the predicted and observed number of solar neutrinos at Earth.

spectrum: See absorption spectrum, continuous spectrum, emission spectrum.

Stefan-Boltzmann law: an empirical relation between a blackbody’s luminosity per unit area and its temperature.

subatomic level: the length-scale beneath that of atoms at which the constituents of atoms exist.

sunspots: dark spots on the Sun where less thermal radiation is emitted due to magnetic phenomena,

temperature: a measure of the amount of heat or thermal energy of a body.

thermal energy: the energy related to an object’s heat.

thermodynamics: a branch of physics used to describe heat and temperature.

tritium: an isotope of hydrogen with two neutrons.

wavelength (λ): The length over which electromagnetic radiation completes one oscillation.

Wien’s law: an empirical law relating the wavelength of peak blackbody radiation to temperature.