Quantum

Question 1

The photoelectric effect is

Answer 1

The emission of electrons from a metal surface when electromagnetic radiation of a sufficiently high frequency is incident on the surface.

Question 2

A photon is

Answer 2

Quantum of energy of electromagnetic radiation, where the energy of a single photon is directly proportional to its frequency, E = hf

Question 3

What are the equations of E

Answer 3

hf
(hc)/Lambda
Work function energy + Max KE
(hc)/(Max lambda) + eVs

Question 4

The work function energy is

Answer 4

The minimum photon energy required to remove an electron from the metal surface

Question 5

The threshold frequency is

Answer 5

Minimum frequency of electromagnetic radiation for emission of electrons from a metal surface.

Question 6

The stopping potential (Vs) is

Answer 6

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

Question 7

Intensity at a beam of photons is

Answer 7

The energy transmitted per unit area per unit time

Question 8

The formulas for intensity of a beam of photons:

Answer 8

P/A
E/(tA)
(NphF)/(tA)

Question 9

An energy level is

Answer 9

The quantised energy corresponding to an allowed state of the electron within the atom

Question 10

The de Broglie wavelength is

Answer 10

The associated wavelength of a moving particle which is dependent on its momentum.

Question 11

The Heisenberg’s uncertainty principle states that

Answer 11

It is impossible to measure the exact position and momentum of a particle at the same time.

Question 12

Appearance of a visible line emission spectrum

Answer 12

Coloured lines on a dark background

Question 13

How does the line spectra provide evidence for discrete energy levels in isolated atoms?

Answer 13

Each line on the line spectrum corresponds to a specific wavelength and frequency of light, which implies that energies of photons emitted when electrons de-excite are discrete.

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 14

What is a continuous spectrum produced by?

Answer 14

Hot solids and liquids, and by high density gases such as the sun.

Has a continuous range of wavelengths.

Question 15

What is an emission line spectrum produced by?

Answer 15

Low density monoatomic gases such as in a gas discharge tube at low pressure.

Question 16

What is an absorption line spectrum produced by?

Answer 16

White light from an incandescent lamp passes through a container of cool gas.

Question 17

What does an absorption line spectrum look like?

Answer 17

Dark lines on a continuous spectrum

Question 18

State 3 pieces of evidence provided by the photoelectric effect for a particulate nature of electromagnetic radiation.

Answer 18

1. The existence of a threshold frequency below for which no emission of photoelectrons takes place.
2. The instantaneous emission of photoelectrons.
3. Max KE of the emitted photoelectrons depends on the frequency of the EM radiation.
4. Max KE of the emitted photoelectrons is independent of the intensity of the EM radiation.
5. Rate of emission of photoelectrons is proportional to the intensity of EM radiation at constant frequency.

Question 19

How does the existence of threshold frequency deviate from predictions of classic wave theory?

Answer 19

The existence of a threshold frequency means photons must have energy greater than the work function energy to remove electrons from the surface. This is because although intensity of the radiation affects the rate of photons per second, each photon still does not have enough energy to overcome the work function energy, which thus deviates from the classic wave theory which predicts that electrons would be emitted at any frequency.

Question 20

How does the existence of instantaneous emission of electrons deviate from predictions of classic wave theory?

Answer 20

The instant. emission of electrons (no time lag) using EM radiation of very low intensities but having a frequency greater than the threshold frequency, deviates from classic wave theory that predicts that there should be a measurable time lag because electrons need to accumulate enough energy before it gets emitted

Question 21

How does the existence of maximum KE of electrons deviate from predictions of classic wave theory?

Answer 21

Max KE of electrons is independent of intensity but dependent on frequency of EM radiation, which deviates from classical wave theory which predicts that increasing the intensity should lead to greater KE and speed of the emitted electrons.
Photons contain a discrete amount of energy. Electrons can absorb all of the photon energy or not at all - cannot accumulate photon energy. (No installments) This explains why there is a minimum frequency needed to release the electrons.

Question 22

Explain why electrons emitted from a metal surface have a range of values for kinetic energy below a maximum value.

Answer 22

For other electrons below the surface, some energy is required to bring the electrons to the surface as these electrons need to overcome a large energy barrier and are also more likely to collide with other atoms while on its way to the surface, so for these electrons they will be emitted with a range of kinetic energies lesser than the maximum value.

Question 23

A spectrum of the light emerging from a cloud of cool gas is viewed using a diffraction grating. Explain why this spectrum contains a number of dark lines.

Answer 23

Electrons within the cool cloud of gas, in their ground state, absorbs the photon energy from the incident white light,
where photon energy corresponds to the energy difference between two discrete energy levels, causing electrons to become excited to a higher energy level.
The excited electrons, being unstable, de-excite to a lower energy level and emit photons in all directions.
Hence these photons of specific frequencies which were absorbed and re-emitted appear as dark lines.

Question 24

High energy electrons collide with a metal target, producing x-ray photons. Explain why there is a continuous distribution of wavelengths.

Answer 24

Bombarding electrons are slowed down when approaching target metal atoms and lose a continuous range of kinetic energies, which are converted to X-ray photons with a continuous range of photon energies. Since E = hc/λ, there is a continuous distribution of wavelengths of X-ray photons emitted.

Question 25

High energy electrons collide with a metal target, producing x-ray photons. Explain why there is a sharp cut-off at short wavelength.

Answer 25

The bombarding electrons that is completely stopped by the target metal atoms loses all its kinetic energy and emits the most energetic x-ray photon. Since E = hc/λ, the X-ray photon emitted will have the shortest wavelength. There can be no more X-ray photons since the most energetic has been cut off already.

Question 26

High energy electrons collide with a metal target, producing x-ray photons. Explain why there is a series of characteristic peaks superimposed on the continuous distribution of wavelengths.

Answer 26

Characteristic peaks are produced when the bombarding electrons knocks out an inner shell electron, causing a vacancy where
an outer shell electron refills that vacancy, emitting an X-ray photon of energy and wavelength corresponding to the energy difference between the two shells.

Question 27

Why do concentric rings provide evidence for the wave nature of particles?

Answer 27

The series of concentric rings is evidence of diffraction and interference, which are behaviours associated with waves.
This is because after passing through the carbon film (which acts as a grating), the electrons spread out and interfere with each other to form bright rings.

Question 28

What would happen to concentric rings if the speed of electrons is gradually increased?

Answer 28

When speed of electrons is increased, momentum is increased
This causes the de Broglie wavelength to decrease. (λ = h/p = h/mv)
Hence angle of diffraction decreases, so there is lesser scattering of electrons, so diameters of concentric rings decreases.

Question 29

A stationary particle decays into 2 gamma photons. Explain why the two photons have the same energy and travel in opposite directions.

Answer 29

Total initial momentum before decay is zero so by conservation of momentum, total final momentum after decay must be zero.
Hence photons must have equal and opposite momentum, hence they must have the same energy (E = hc/λ = pc) and travel in opposite directions.

Question 30

Explain why when ultraviolet light is incident on the surface of a zinc plate, electrons are ejected but none are ejected when the plate is illuminated with red light.

Answer 30

For photoelectric effect and emission of photoelectrons to occur, the energy ‘hf’ of the incident photon must be greater than the work function of the metal ‘hfo’, where fo is the threshold frequency.

Since UV light had higher frequency f, the incident photons have energy greater than the work function energy of the metal, so emission of photoelectrons is possible.

Since red light has a frequency lower than F, energy is lesser than w.f.e. of metal and emission is not possible.

Question 31

A source emitting high energy beta particles is placed in an aluminum container. Why is X ray radiation emitted from the container walls although the beta particles are absorbed?

Answer 31

High energy beta particles emitted during the process are high speed electrons.

These electrons bombard and collide with the aluminum atoms and are slowed down, losing kinetic energy which gets converted to X-ray photons. They may also knock out inner shell electrons in the aluminum, causing outer shell electrons to fill the vacancy and emit X-ray photons in the process.

The higher energy x-ray photons are able to penetrate the aluminum walls and are thus emitted.

Question 32

Explain why scattering light or gamma rays bouncing off an electron causes the electron to change its momentum.

Answer 32

Photons have momentum. When photons bounce off the electron, a collision occurs and momentum is transferred to the electron (by POCOM), causing the electron’s momentum to change.

Question 33

By reference to how the momentum change of an electron depends on the wavelength of the electromagnetic radiation used to observe it, explain why using gamma rays could allow a more precise measurement of position to be made.

Answer 33

(Since p = h/λ) shorter wavelength of EM radiation leads to greater momentum transfer and change imparted to the electron,
(By ΔxΔp >= h) so there is greater uncertainty in momentum Δp so uncertainty in position Δx is minimised (allowing for more precise measurements to be made).

Question 34

Define threshold frequency with reference to the photoelectric effect

Answer 34

The minimum frequency of incident photons where photons are emitted from the surface

Question 35

Explain origin of continuous background spectrum

Answer 35

Photons are produced whenever a charged particle such as electrons decelerates. The electrons produced from the cathode will strike the metal target and decelerate. Since the electrons have a continuous distribution of decelerations, the x ray photons produced also have a continuous distribution of wavelengths.