E.2 Quantum physics questions for IB Physics

65 questions on Quantum physics, written by Augment in the style of IB exam papers. Six of them are below in full, to answer on paper.

IB Physics
Part of E Nuclear and quantum physics
HL only

Questions by paper and level

Paper	Questions
Paper 1A	31
Paper 2	34
Every paper	65

Counted on 10 November 2026

Questions to try

Written by Augment in the style of IB exams, not past papers. Answer them on paper, then check each one against its markscheme with a free account.

Question 1 · PHY-00005Paper 1A · 1 mark

Monochromatic light is shone on a metal surface and photoelectrons are emitted. The intensity of the light is doubled while its frequency is kept constant. What happens to the maximum kinetic energy of the photoelectrons and to the photoelectric current?

A
Maximum kinetic energy doubles; current is unchanged.
B
Maximum kinetic energy is unchanged; current doubles.
C
Both double.
D
Both are unchanged.

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Question 2 · PHY-00973Paper 2 · 12 marks

A light-beam alarm uses a photocell. Light of wavelength 450 nm falls on the metal cathode of the photocell, which has a work function of 2.1 eV. The power of the light reaching the cathode is 2.0 mW. On average, 8.0 % of the incident photons each release one electron that reaches the anode.

(a)
Show that the energy of one photon is about 2.8 eV.
2 marks
(b)
Calculate the number of photons incident on the cathode each second.
2 marks
(c)
Calculate the current in the photocell.
2 marks
(d)
The photocell is connected in series with a 47 kΩ resistor and a battery. Calculate the potential difference across the resistor.
1 mark
(e)
Determine the stopping potential for the electrons emitted from the cathode.
2 marks
(f)
The intensity of the light is halved, but its wavelength is unchanged. State and explain the effect on the potential difference across the resistor and on the stopping potential.
3 marks

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Question 3 · PHY-01039Paper 1A · 1 mark

A clean zinc plate is attached to a negatively charged electroscope. When the plate is illuminated by a weak ultraviolet lamp, the electroscope discharges at once. When it is illuminated by a very bright red lamp instead, the electroscope does not discharge, however long the lamp is left on. What explains the result with the red lamp?

A
The red lamp delivers less energy per second to the plate than the ultraviolet lamp.
B
Electrons in the zinc need more time to absorb enough energy from red light.
C
Red light is reflected by the zinc surface, while ultraviolet radiation is absorbed.
D
The energy of one photon of red light is less than the work function of zinc.

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Question 4 · PHY-01227Paper 2 · 6 marks

Louis de Broglie suggested that the electron in a hydrogen atom behaves as a wave and that an allowed orbit of radius $r$ is one whose circumference is a whole number $n$ of de Broglie wavelengths $λ$ , so that $2 π r = n λ$ .

(a)
Show that this condition leads to Bohr's condition $m_{e} v r = \frac{n h}{2 π}$ for an electron of mass $m_{e}$ moving with speed $v$ .
2 marks
(b)
The radius of the $n = 2$ orbit is $2.12 \times 10^{- 10}$ m. Calculate the de Broglie wavelength of the electron in this orbit.
1 mark
(c)
Hence determine the speed of the electron in the $n = 2$ orbit.
2 marks
(d)
Outline one reason why the Bohr model is not a complete description of atoms.
1 mark

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Question 5 · PHY-01040Paper 1A · 1 mark

Ultraviolet radiation with a photon energy of 6.2 eV is incident on a clean tungsten surface. The work function of tungsten is 4.5 eV. What is the stopping voltage for the photoelectrons?

A
1.7 V
B
4.5 V
C
6.2 V
D
10.7 V

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Question 6 · PHY-01238Paper 2 · 11 marks

A beam of high-energy neutrons is directed at a thin sample containing oxygen-16 nuclei. The de Broglie wavelength of the neutrons is $2.6 \times 10^{- 15}$ m. A detector measures the intensity of the neutrons scattered at different angles $θ$ to the direction of the beam.

Graph of scattered neutron intensity (logarithmic scale, 0.01 to 200, arbitrary units) against scattering angle in degrees (0 to 50). The intensity falls from about 95 at 3 degrees to a sharp minimum at 25 degrees, rises to a small secondary maximum of about 5 near 37 degrees and then falls again.

The graph shows the variation of the intensity of the scattered neutrons with $θ$ .

(a)
Calculate the momentum of a neutron in the beam.
1 mark
(b)
The first minimum of intensity occurs at an angle $θ$ given by $\sin θ \approx \frac{λ}{D}$ , where $D$ is the diameter of the nucleus. Estimate, using the graph, the radius of an oxygen-16 nucleus.
2 marks
(c)
Hence determine a value for $R_{0}$ in $R = R_{0} A^{\frac{1}{3}}$ .
2 marks
(d)
The oxygen sample is replaced by a thin lead-208 foil and the same neutron beam is used. Predict the angle of the first minimum.
2 marks
(e)
Outline why this experiment shows that neutrons have wave properties.
2 marks
(f)
Suggest why neutrons with a de Broglie wavelength of about $10^{- 10}$ m could not be used to measure the size of a nucleus.
1 mark
(g)
Suggest one advantage of using neutrons rather than protons of the same de Broglie wavelength.
1 mark

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