Modern physics Flashcards
Threshold frequency
is the minimum-frequency radiation that is required to release electrons from the surface of a metal.
Work function
is the minimum energy, or minimum work, needed to release electrons from the surface of a metal.
Valence band
is the range of energy levels in which valence electrons are situated when in their lowest energy state.
Conduction band
is the range of energy levels into which electrons are excited and in which they are free to move around the material.
Forbidden gap or band gap
is the energy gap between the valence and conduction bands, in which electrons cannot exist.
Semiconductors
are materials whose resistivity is between that of insulators and metallic conductors. They differ from conductors, not only in their higher resistance but also in the conduction process.
Positive hole
is the vacancy left in the valence band when an electron is excited into the conduction band.
Thermionic emission
is the emission of electrons from a hot metal surface.
Contrast
is a measure of the difference in brightness between light and dark areas.
Contrast medium
is a material that is a good absorber of X-rays, which consequently improves the contrast of an image.
Half-value thickness
of a material is the thickness that reduces the intensity of the incident signal to half its original intensity.
Binding energy
of a nuclide is the work done or energy required to separate, to infinity, the nucleus into its constituent protons and neutrons.
Mass defect
of a nuclide is the difference in mass between the nucleus of a nuclide and the total mass of the nucleons of that nuclide, when separated to infinity.
Activity
of a sample of radioactive material is the number of decays per unit time.
Decay constant
is the probability per unit time that a nucleus will decay.
