CDS Physics · Free notes
Modern Physics: CDS Physics
Modern Physics has 15 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 4 EASY, 10 MODERATE and 1 HARD. Almost all of them are recall: what X-rays are and what they do, what each kind of decay changes, how fission differs from fusion. One fact has been asked three times: X-rays carry no charge, so neither an electric nor a magnetic field bends them. Learn the particle table and the decay table exactly; the only sum asked so far turns a photon's momentum into its energy.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Photons and X-rays
7 PYQsLight as photons with energy and momentum, atoms that absorb and give out exact frequencies (the atomic clock), and what X-rays are, what they do and what they are used for.
Atoms and Nuclei
8 PYQsThe particles inside and beyond the atom, the three kinds of radioactive decay and the fallout isotopes, and how fission and fusion release energy in reactors, bombs and the Sun.
Formula & revision sheet
2 formulas · 3 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
2 formulas · 3 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (1)
X-rays: what they are and what they do6 rows
| Property or use | Fact |
|---|---|
| Nature | Electromagnetic waves with no charge |
| Electric and magnetic fields | Do not deflect X-raysQ Asked in 2020 (I), 2025 (II) and 2026 (I): neither an electric nor a magnetic field bends X-rays. |
| Wavelength | About 0.01 to 10 nm, much shorter than visible light |
| Penetration | Pass through flesh; stopped by bone and metal |
| Metal objects during a scan | Block or reflect the beam, leaving a bright or dark patch that hides the body partQ CDS 2026 (II): metal is removed because it spoils the image. |
| Welds, castings and crystals | Show hidden cracks; reveal how atoms are arrangedQ CDS 2024 (II): checking welded joints and studying crystals are both uses. |
Watch out for (4)
- For a photon, energy is momentum times c→ Photons: energy, momentum and atomic energy levels
- Shorter wavelength means more energy→ Photons: energy, momentum and atomic energy levels
- No charge, no bending→ X-rays: what they are and what they do
- Metal is removed to save the picture→ X-rays: what they are and what they do
Reference tables (2)
Subatomic particles6 rows
| Particle | Charge | Mass and where it is found |
|---|---|---|
| Electron | Negative (−1) | About 1/1836 of a proton; moves round the nucleus |
| Proton | Positive (+1) | About 1 u (938 MeV); in the nucleus |
| Neutron | None | Slightly more than a proton; in the nucleusQ CDS 2020 (II): neutrons are inside the nucleus; it is electrons that move round it. |
| Muon | Negative (−1) | About 207 times an electron; made by cosmic rays, lives about 2 millionths of a secondQ CDS 2024 (I): the electron, proton, neutron and muon are all subatomic particles. |
| Photon | None | No rest mass; carries light and all electromagnetic radiation |
| Higgs boson | None | About 125 GeV; found at CERN's Large Hadron Collider in 2012Q CDS 2024 (I): of 0.5 MeV, 900 MeV, 100 GeV and 1000 GeV, the closest to its mass is 100 GeV. |
Radioactive decay and fallout3 rows
| Decay | What leaves the nucleus | Change in Z and A |
|---|---|---|
| Alpha | A helium nucleus (2 protons and 2 neutrons) | Z falls by 2, A falls by 4 |
| Beta (minus) | An electron, made when a neutron turns into a proton | Z rises by 1, A unchanged |
| Gamma | A high-energy photon | No change in Z or AQ CDS 2016 (II): in gamma decay neither the mass number nor the atomic number changes. |
Watch out for (8)
- Neutrons stay in the nucleus→ Subatomic particles
- The Higgs boson is about 125 GeV, not MeV→ Subatomic particles
- Gamma changes no count→ Radioactive decay and fallout
- Beta decay raises the atomic number→ Radioactive decay and fallout
- All three fallout isotopes travel through food→ Radioactive decay and fallout
- Fission splits, fusion joins→ Fission, fusion and nuclear energy
- A reactor has a chain reaction too, kept under control→ Fission, fusion and nuclear energy
- Moderator slows, control rods absorb→ Fission, fusion and nuclear energy
PYQ weightage by concept
5 concepts · 15 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
5 concepts · 15 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| X-rays: what they are and what they do | 5 | 33% |
| Photons: energy, momentum and atomic energy levels | 2 | 13% |
| Concept | PYQs | Share |
|---|---|---|
| Subatomic particles | 3 | 20% |
| Fission, fusion and nuclear energy | 3 | 20% |
| Radioactive decay and fallout | 2 | 13% |
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.