Physics · Unit 18
Modern Physics
Atoms, quanta and relativity
By 1900 physics looked close to finished. Within thirty years it had been rebuilt twice, because two sets of experiments could not be made to fit.
The unit covers atomic structure, nuclear physics and radioactivity, the photoelectric effect and quantum ideas, special relativity, and particle physics.
This unit breaks down into 20 short steps and 120 questions, starting at difficulty 2 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.
- Steps
- 20
- Questions
- 120
- Difficulty
- 2-3
What this unit covers
- Particle Physics
- Nuclear Physics and Radioactivity
- Quantum and the Photoelectric Effect
- Atomic Structure
- Special Relativity
Where this fits
The last and hardest unit in the Physics track. Assumes most of the others.
Where people slip
Quantum and relativistic effects are not wrong at everyday scales - they are present and far too small to notice. Classical physics is the approximation, not the rule being broken.
How the unit is structured
Modern Physics runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 2 and climb to 3 as you progress.
Challenge rounds
Example questions
30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.
Atomic Structure
- Fill the blankLevel 2
1. The dense central core of an atom is called the ____.
- nucleuscorrect
- electron
- molecule
- shell
The nucleus is the atom's dense core, holding its protons and neutrons.
- Guess the numberLevel 2
2. How many protons does a carbon atom have?
Answer: 6 protons
Carbon has an atomic number of 6, meaning every carbon atom has 6 protons.
- Match the pairsLevel 2
3. Match each part of the atom to its description.
Answer: Proton = Positive particle in the nucleus; Neutron = Neutral particle in the nucleus; Electron = Negative particle orbiting the nucleus; Nucleus = Dense centre of the atom
Protons and neutrons sit in the nucleus, while electrons orbit around it.
- Multiple choiceLevel 2
4. Which particle inside an atom carries a positive electric charge?
- Protoncorrect
- Electron
- Neutron
- Photon
Protons carry a positive charge and are found packed inside the nucleus.
- Choose all that applyLevel 3
5. Which statements about atomic spectra are correct?
- An emission spectrum shows bright lines where atoms release photonscorrect
- An absorption spectrum shows dark lines within a continuous spectrumcorrect
- Each element has its own unique set of spectral linescorrect
- Spectral lines prove that electron energies are continuous
Emission gives bright lines, absorption gives dark lines, and each element has a unique set of lines; the lines show energies are discrete, not continuous.
- Fact or fibLevel 3
6. In the Bohr model, an electron can take any energy value it likes between two allowed energy levels.
Answer: False
Electron energies in an atom are quantized, so only the specific allowed levels exist, never values in between.
Nuclear Physics and Radioactivity
- Guess the numberLevel 2
7. A sample has a half-life of 10 years. After how many years does only one quarter remain?
Answer: 20 years
After one half-life half remains, and after two half-lives (20 years) a quarter remains.
- Match the pairsLevel 2
8. Match each nuclear term to its description.
Answer: Alpha = Helium nucleus stopped by paper; Beta = Fast-moving electron; Gamma = High-energy wave with no charge; Half-life = Time for half the nuclei to decay
Alpha, beta, and gamma are the three main radiations, and half-life measures decay rate.
- Multiple choiceLevel 2
9. Which type of radiation is a helium nucleus made of two protons and two neutrons?
- Alphacorrect
- Beta
- Gamma
- X-ray
An alpha particle is a helium nucleus containing two protons and two neutrons.
- Put in orderLevel 2
10. Order these radiations from least to most penetrating.
Answer: Alpha -> Beta -> Gamma
Alpha is the least penetrating, beta more so, and gamma penetrates the most.
- Sort into groupsLevel 2
11. Sort each radiation by whether it carries an electric charge.
Answer: Alpha particle = Charged; Beta particle = Charged; Gamma ray = Uncharged; Neutron = Uncharged
Alpha and beta particles are charged, while gamma rays and neutrons are uncharged.
- Fill the blankLevel 3
12. When balancing a nuclear decay equation, the total nucleon number and the total ____ must be equal on both sides.
- chargecorrect
- kinetic energy
- wavelength
- volume
Nucleon number and charge (proton number) are both conserved, so both sides of a nuclear equation must balance for them.
Particle Physics
- Fact or fibLevel 2
13. An electron is an example of a lepton.
Answer: True
The electron is a lepton, a fundamental particle that is not made of quarks.
- Match the pairsLevel 2
14. Match each fundamental force to what it does.
Answer: Gravity = Holds planets in orbit; Electromagnetic force = Acts between electric charges; Strong force = Binds quarks inside the nucleus; Weak force = Causes some radioactive decay
The four fundamental forces are gravity, electromagnetism, the strong force, and the weak force.
- Multiple choiceLevel 2
15. Protons and neutrons are each built from smaller particles called what?
- Quarkscorrect
- Leptons
- Photons
- Ions
Protons and neutrons are made of quarks, held together by the strong force.
- Choose all that applyLevel 3
16. Which of these are among the four fundamental forces of nature? Select all that apply.
- Gravitycorrect
- Electromagnetic forcecorrect
- Strong nuclear forcecorrect
- Friction
The four fundamental forces are gravity, electromagnetism, the strong nuclear force and the weak nuclear force; friction is just an everyday effect of electromagnetism.
- Fill the blankLevel 3
17. The ____ boson, discovered at CERN in 2012, is linked to the field that gives fundamental particles their mass.
- Higgscorrect
- gluon
- photon
- W
The Higgs boson is the quantum of the Higgs field, which gives many fundamental particles their mass.
- Guess the numberLevel 3
18. The cosmic microwave background is the cooled afterglow of the Big Bang, filling all of space. What is its temperature today?
Answer: 2.7 K
The cosmic microwave background has a temperature of about 2.7 K, strong evidence for a hot early universe.
Quantum and the Photoelectric Effect
- Build the sentenceLevel 2
19. Arrange the words to state how a photon's energy depends on the light.
Answer: Higher frequency light carries more energy
A photon's energy is proportional to its frequency, so higher-frequency light carries more energy.
- Choose all that applyLevel 2
20. Which statements about the photoelectric effect are correct?
- Higher-frequency light can give electrons more energycorrect
- Below a threshold frequency, no electrons are released
- Brighter light above the threshold releases more electronscorrect
- Electrons escape no matter how low the frequency
There is a threshold frequency below which no electrons escape, and above it brighter light frees more electrons.
- Fact or fibLevel 2
21. A photon is a tiny packet, or quantum, of light energy.
Answer: True
Light energy comes in discrete packets called photons, or quanta.
- Fill the blankLevel 2
22. The idea that energy comes in tiny discrete packets is the basis of ____ physics.
- quantumcorrect
- classical
- nuclear
- thermal
Quantum physics describes energy and matter in terms of discrete packets called quanta.
- Multiple choiceLevel 2
23. In the photoelectric effect, light shining on a metal surface can knock out which particles?
- Electronscorrect
- Protons
- Neutrons
- Atoms
The photoelectric effect is the ejection of electrons from a metal when light strikes it.
- Guess the numberLevel 3
24. An electron (mass 9.1x10^-31 kg) moves at 7.3x10^6 m/s. Using lambda = h/(mv) with h=6.63x10^-34 J s, estimate its de Broglie wavelength in picometres.
Answer: 100 pm
p = mv = 6.65x10^-24 kg m/s, so lambda = 6.63x10^-34 / 6.65x10^-24 = about 1.0x10^-10 m, roughly 100 pm.
Special Relativity
- Build the sentenceLevel 2
25. Arrange the words into a key postulate of special relativity.
Answer: Light always travels at the same speed
One postulate of relativity is that light travels at the same speed for every observer.
- Choose all that applyLevel 2
26. Which of these are predictions of special relativity?
- Moving clocks run slowcorrect
- Moving objects contract along their motioncorrect
- Nothing can exceed the speed of lightcorrect
- Mass has no connection to energy
Special relativity predicts time dilation, length contraction, and a cosmic speed limit set by light.
- Fill the blankLevel 2
27. Einstein published his special theory of relativity in the year ____.
- 1905correct
- 1687
- 1969
- 2000
Albert Einstein published the special theory of relativity in 1905.
- Guess the numberLevel 2
28. What is the approximate speed of light in a vacuum, in km/s?
Answer: 300000 km/s
Light travels through a vacuum at about 300,000 km/s.
- Multiple choiceLevel 2
29. According to special relativity, nothing can travel faster than what?
- The speed of lightcorrect
- The speed of sound
- The Earth's rotation
- A jet aeroplane
The speed of light in a vacuum, about 300,000 km/s, is the universe's ultimate speed limit.
- Fact or fibLevel 3
30. No object with mass can be accelerated all the way up to the speed of light in a vacuum.
Answer: True
As speed approaches c the energy needed grows without limit, so a massive object can never actually reach the speed of light.
Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.
How you practise
This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.
- Build the sentence
- Choose all that apply
- Fact or fib
- Fill the blank
- Guess the number
- Listen and choose
- Match the pairs
- Multiple choice
- Odd one out
- Picture question
- Put in order
- Sequence recall
- Sort into groups
- Spell it
- Tap the pairs
- True or false
- Type the answer
Practise Modern Physics
120 questions across 20 steps. Start with step one and crawl at your own pace.
Play this unitRead about Modern Physics
Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.
- What Is Nuclear Fission? Splitting the Atom and What Comes OutFission is the splitting of a heavy nucleus into two lighter ones, releasing energy and neutrons that split more. How it was found, and how a reactor tames it.September 14, 2026 · 5 min read
- What Is Radioactivity? Unstable Nuclei and Radiation ExplainedLearn why unstable atomic nuclei decay, what alpha, beta, and gamma radiation are, and how half-life describes decay.August 14, 2026 · 6 min read
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- Energy and Its FormsThe power to make things happen
- Light and SoundSeeing and hearing the world
- Electricity and MagnetsSparks, circuits and magnets