CDS Physics · Formula sheet
Modern Physics formulas
2 formulas, 3 reference tables and 12 common traps for CDS Physics Modern Physics, grouped by subtopic.
Photons and X-rays
Learn this subtopic in the notesPhotons: energy, momentum and atomic energy levels
Photon energy and momentum
- Planck's constant, 6.63 × 10⁻³⁴ J s
- frequency of the light
- wavelength
- momentum of the photon
X-rays: what they are and what they do
| 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. |
Common traps
For a photon, energy is momentum times c
A photon has no rest mass, so does not apply. Use : a momentum in MeV/ gives the same number in MeV.
Shorter wavelength means more energy
: halving the wavelength doubles the energy. X-rays and gamma rays carry far more energy per photon than visible light.
No charge, no bending
X-rays are electromagnetic waves, not charged particles. A field bends a beam of electrons, but neither an electric nor a magnetic field bends X-rays.
Metal is removed to save the picture
Metal blocks X-rays and leaves a patch on the image that hides the body part. It does not melt, spark or get changed by the X-rays.
Atoms and Nuclei
Learn this subtopic in the notesFission, fusion and nuclear energy
Mass and energy
- mass lost in the reaction (mass defect)
- speed of light, 3 × 10⁸ m/s
Subatomic particles
| 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 fallout
| 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. |
Common traps
Neutrons stay in the nucleus
Only electrons move round the nucleus. Neutrons and protons are packed inside it.
The Higgs boson is about 125 GeV, not MeV
Its mass is about 125 GeV, more than 100 times a proton's (about 0.94 GeV). Do not pick an MeV value because the number looks familiar.
Gamma changes no count
A gamma ray is a photon: no charge, no mass. The nucleus only loses energy, so Z and A stay the same.
Beta decay raises the atomic number
In beta decay a neutron becomes a proton and an electron leaves. So Z goes up by 1 while A is unchanged. It does not go down.
All three fallout isotopes travel through food
Strontium-90, iodine-131 and caesium-137 all enter the body through the food chain, each settling where a similar element normally goes.
Fission splits, fusion joins
Fission breaks a heavy nucleus into lighter ones. Fusion joins light nuclei into a heavier one. Joining two nuclei is never fission.
A reactor has a chain reaction too, kept under control
Both a reactor and an atom bomb run a fission chain reaction. The reactor keeps it controlled with a moderator and control rods; the bomb lets it run away.
Moderator slows, control rods absorb
The moderator (heavy water, graphite) slows neutrons so they can split uranium-235. The control rods (cadmium, boron) absorb neutrons to keep the rate steady. Do not swap them.