Chapter 19: Quantum Physics

Question 1

What is the photoelectric effect?

Answer 1

The photoelectric effect is the emission of photoelectrons from a metal surface when
electromagnetic radiation of a sufficiently high frequency is incident on it.

Question 2

What is a photon?

Answer 2

A photon is a quantum of energy of electromagnetic radiation.

Question 3

State what is meant by the work function energy of a metal

Answer 3

The work function energy Φ of a metal is the minimum amount of energy that will remove an electron from the metal surface.

Question 4

State what is meant by threshold frequency

Answer 4

The threshold frequency f0 is the minimum frequency of an incident photon that will remove an electron from the metal surface.

Question 5

Define stopping potential

Answer 5

The stopping potential Vs is the minimum potential difference between the metal and
collector that will prevent the most energetic photoelectron from reaching the collector, resulting in zero photocurrent.

Question 6

What does line spectra provide evidence for?

Answer 6

• Each line on the line spectrum corresponds to a single wavelength and frequency of light, which implies that the photon energy is discrete (a specific value).
• Since energy of the emitted photon corresponds to the difference in energy levels in the atom, this implies that the energy levels must be discrete.

Question 7

Define energy level

Answer 7

An energy level is the quantised energy corresponding to an allowed state (or orbital) of the electron within the atom.

Question 8

What is a continuous spectrum?

Answer 8

A continuous spectrum is produced by hot
solids and liquids, and by high-density gases such as the Sun. Spectra of this type consists of a continuous range of wavelengths.

Question 9

What is an emission spectrum?

Answer 9

An emission line spectrum is produced by
low-density monatomic gases such as in a gas discharge tube at low pressure. Spectra of this type consists of coloured lines on a dark background.

Question 10

What is an absorption line spectrum?

Answer 10

An absorption line spectrum is produced
when white light from an incandescent lamp passes through a container of cool gas. Spectra of this type appear as dark lines on a continuous spectrum.

Question 11

What is the de Broglie Wavelength?

Answer 11

The de Broglie wavelength is the wavelength associated with a moving particle.

Question 12

State the Heisenberg Uncertainty Principle

Answer 12

The Heisenberg Uncertainty Principle states that it is impossible to measure the exact
position and momentum at the same time (i.e. cannot both be zero).