### Key Terms

- absorber
- any object that absorbs radiation

- absorption spectrum
- wavelengths of absorbed radiation by atoms and molecules

- Balmer formula
- describes the emission spectrum of a hydrogen atom in the visible-light range

- Balmer series
- spectral lines corresponding to electron transitions to/from the $n=2$ state of the hydrogen atom, described by the Balmer formula

- blackbody
- perfect absorber/emitter

- blackbody radiation
- radiation emitted by a blackbody

- Bohr radius of hydrogen
- radius of the first Bohr’s orbit

- Bohr’s model of the hydrogen atom
- first quantum model to explain emission spectra of hydrogen

- Brackett series
- spectral lines corresponding to electron transitions to/from the $n=4$ state

- Compton effect
- the change in wavelength when an X-ray is scattered by its interaction with some materials

- Compton shift
- difference between the wavelengths of the incident X-ray and the scattered X-ray

- Compton wavelength
- physical constant with the value ${\lambda}_{c}=2.43\phantom{\rule{0.2em}{0ex}}\text{pm}$

- cut-off frequency
- frequency of incident light below which the photoelectric effect does not occur

- cut-off wavelength
- wavelength of incident light that corresponds to cut-off frequency

- Davisson–Germer experiment
- historically first electron-diffraction experiment that revealed electron waves

- de Broglie wave
- matter wave associated with any object that has mass and momentum

- de Broglie’s hypothesis of matter waves
- particles of matter can behave like waves

- double-slit interference experiment
- Young’s double-slit experiment, which shows the interference of waves

- electron microscopy
- microscopy that uses electron waves to “see” fine details of nano-size objects

- emission spectrum
- wavelengths of emitted radiation by atoms and molecules

- emitter
- any object that emits radiation

- energy of a photon
- quantum of radiant energy, depends only on a photon’s frequency

- energy spectrum of hydrogen
- set of allowed discrete energies of an electron in a hydrogen atom

- excited energy states of the H atom
- energy state other than the ground state

- Fraunhofer lines
- dark absorption lines in the continuum solar emission spectrum

- ground state energy of the hydrogen atom
- energy of an electron in the first Bohr orbit of the hydrogen atom

- group velocity
- velocity of a wave, energy travels with the group velocity

- Heisenberg uncertainty principle
- sets the limits on precision in simultaneous measurements of momentum and position of a particle

- Humphreys series
- spectral lines corresponding to electron transitions to/from the $n=6$ state

- hydrogen-like atom
- ionized atom with one electron remaining and nucleus with charge $+Ze$

- inelastic scattering
- scattering effect where kinetic energy is not conserved but the total energy is conserved

- ionization energy
- energy needed to remove an electron from an atom

- ionization limit of the hydrogen atom
- ionization energy needed to remove an electron from the first Bohr orbit

- Lyman series
- spectral lines corresponding to electron transitions to/from the ground state

- nuclear model of the atom
- heavy positively charged nucleus at the center is surrounded by electrons, proposed by Rutherford

- Paschen series
- spectral lines corresponding to electron transitions to/from the $n=3$ state

- Pfund series
- spectral lines corresponding to electron transitions to/from the $n=5$ state

- photocurrent
- in a circuit, current that flows when a photoelectrode is illuminated

- photoelectric effect
- emission of electrons from a metal surface exposed to electromagnetic radiation of the proper frequency

- photoelectrode
- in a circuit, an electrode that emits photoelectrons

- photoelectron
- electron emitted from a metal surface in the presence of incident radiation

- photon
- particle of light

- Planck’s hypothesis of energy quanta
- energy exchanges between the radiation and the walls take place only in the form of discrete energy quanta

- postulates of Bohr’s model
- three assumptions that set a frame for Bohr’s model

- power intensity
- energy that passes through a unit surface per unit time

- propagation vector
- vector with magnitude $2\pi \phantom{\rule{0.1em}{0ex}}\text{/}\phantom{\rule{0.1em}{0ex}}\lambda $ that has the direction of the photon’s linear momentum

- quantized energies
- discrete energies; not continuous

- quantum number
- index that enumerates energy levels

- quantum phenomenon
- in interaction with matter, photon transfers either all its energy or nothing

- quantum state of a Planck’s oscillator
- any mode of vibration of Planck’s oscillator, enumerated by quantum number

- reduced Planck’s constant
- Planck’s constant divided by $2\pi $

- Rutherford’s gold foil experiment
- first experiment to demonstrate the existence of the atomic nucleus

- Rydberg constant for hydrogen
- physical constant in the Balmer formula

- Rydberg formula
- experimentally found positions of spectral lines of hydrogen atom

- scattering angle
- angle between the direction of the scattered beam and the direction of the incident beam

- Stefan–Boltzmann constant
- physical constant in Stefan’s law

- stopping potential
- in a circuit, potential difference that stops photocurrent

- wave number
- magnitude of the propagation vector

- wave quantum mechanics
- theory that explains the physics of atoms and subatomic particles

- wave-particle duality
- particles can behave as waves and radiation can behave as particles

- work function
- energy needed to detach photoelectron from the metal surface

- $\alpha $-particle
- doubly ionized helium atom

- $\alpha $-ray
- beam of $\alpha $-particles (alpha-particles)

- β-ray
- beam of electrons

- γ-ray
- beam of highly energetic photons