Modern physics

Question 1

Threshold frequency

Answer 1

is the minimum-frequency radiation that is required to release electrons from the surface of a metal.

Question 2

Work function

Answer 2

is the minimum energy, or minimum work, needed to release electrons from the surface of a metal.

Question 3

Valence band

Answer 3

is the range of energy levels in which valence electrons are situated when in their lowest energy state.

Question 4

Conduction band

Answer 4

is the range of energy levels into which electrons are excited and in which they are free to move around the material.

Question 5

Forbidden gap or band gap

Answer 5

is the energy gap between the valence and conduction bands, in which electrons cannot exist.

Question 6

Semiconductors

Answer 6

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.

Question 7

Positive hole

Answer 7

is the vacancy left in the valence band when an electron is excited into the conduction band.

Question 8

Thermionic emission

Answer 8

is the emission of electrons from a hot metal surface.

Question 9

Contrast

Answer 9

is a measure of the difference in brightness between light and dark areas.

Question 10

Contrast medium

Answer 10

is a material that is a good absorber of X-rays, which consequently improves the contrast of an image.

Question 11

Half-value thickness

Answer 11

of a material is the thickness that reduces the intensity of the incident signal to half its original intensity.

Question 12

Binding energy

Answer 12

of a nuclide is the work done or energy required to separate, to infinity, the nucleus into its constituent protons and neutrons.

Question 13

Mass defect

Answer 13

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.

Question 14

Activity

Answer 14

of a sample of radioactive material is the number of decays per unit time.

Question 15

Decay constant

Answer 15

is the probability per unit time that a nucleus will decay.

Question 16

1 bequerel (Bq)

Answer 16

is an activity of 1 decay per second.

Question 17

Half-life

Answer 17

of a radioactive isotope is the time taken for half of the nuclei of that isotope in any sample to decay.