Waves and Radiation and nuclear test revision
Wave
A wave is a disturbance that transfers energy from one point to another without transferring matter.
Particles in the medium vibrate around a fixed position as energy moves through.
Example: Sound waves in air, ripples on water, or light waves.
2⃣ Mechanical vs Electromagnetic Waves
Mechanical waves require a medium (solid, liquid, or gas) to transfer energy — e.g. sound or water waves.
Electromagnetic waves do not need a medium and can travel through a vacuum — e.g. light, radio, X-rays.
3⃣ Transverse Waves
Particles move perpendicular to the direction the wave travels.
They have crests and troughs.
Examples: Light waves, waves on a rope.
4⃣ Longitudinal Waves
Particles move parallel to the wave’s direction.
They form compressions (high pressure) and rarefactions (low pressure).
Example: Sound waves in air.
5⃣ Amplitude (A)
Maximum displacement from equilibrium position.
Related to energy — higher amplitude = higher energy.
Example: Louder sounds or brighter light have greater amplitude.
6⃣ Wavelength (λ)
Distance between two points that are in phase (e.g. crest to crest or compression to compression).
Measured in metres (m).
7⃣ Frequency (f)
Number of complete wave cycles per second, measured in hertz (Hz).
Formula: f=1Tf = \frac{1}{T}f=T1.
Example: A 10 Hz wave completes 10 vibrations every second.
8⃣ Period (T)
Time taken for one complete wave to pass a point.
Inverse of frequency: T=1fT = \frac{1}{f}T=f1.
9⃣ Wave Speed (v)
Rate at which the wave travels through a medium.
Formula: v=fλv = f\lambdav=fλ.
Example: Sound in air ≈ 330 m/s.
10⃣ Reflection
Occurs when a wave bounces off a surface.
Law of reflection: Angle of incidence = angle of reflection.
Example: Echoes or light reflecting off a mirror.
11⃣ Refraction
When a wave enters a medium with different density, its speed and direction change.
Snell’s Law: n1sinθ1=n2sinθ2n_1 \sin\theta_1 = n_2 \sin\theta_2n1sinθ1=n2sinθ2.
Example: Light bends when moving from air into glass.
12⃣ Diffraction
The bending or spreading of waves when passing through a narrow gap or around edges.
Greatest when the gap size ≈ wavelength.
Example: Sound spreading around a doorway.
13⃣ Superposition
When two or more waves overlap, their displacements add together at each point.
In-phase → constructive interference (amplitudes add).
Out-of-phase → destructive interference (amplitudes cancel).
14⃣ Standing Wave
A stationary pattern formed when two identical waves move in opposite directions and interfere.
Has nodes (no motion) and antinodes (maximum motion).
Common in strings or air columns.
15⃣ Harmonics
Integer multiples of the fundamental frequency.
fn=nf1f_n = n f_1fn=nf1.
For strings/open pipes: n = 1, 2, 3, …
For closed pipes: n = 1, 3, 5, …
16⃣ Overtones
The higher resonant frequencies above the fundamental.
1st overtone = 2nd harmonic (open or string), or 3rd harmonic (closed pipe).
17⃣ Resonance
When an object is driven at its natural frequency, resulting in maximum amplitude vibrations.
Example: Swing pushed at just the right rhythm, or glass shattering from a singer’s note.
18⃣ Intensity (I)
Power of a wave per unit area.
I=PAI = \frac{P}{A}I=AP.
Follows inverse square law: I∝1r2I \propto \frac{1}{r^2}I∝r21.
19⃣ Coherent Sources
Two sources that emit waves with the same frequency and a constant phase difference.
Required for stable interference patterns.
Example: Two slits in Young’s double-slit experiment.
20⃣ Pitch
The perceived frequency of a sound.
High frequency = high pitch.
Low frequency = low pitch.
21⃣ Loudness
The perceived intensity of a sound, related to amplitude.
22⃣ Electromagnetic Spectrum
Range of electromagnetic waves from lowest to highest frequency:
Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → GammaAs frequency ↑, wavelength ↓, energy ↑.
RADIATION & NUCLEAR — Extended Theory Notes
1⃣ Atom
The smallest particle of an element that retains its chemical properties.
Consists of a nucleus (protons + neutrons) surrounded by electrons.
2⃣ Isotope
Atoms of the same element with the same number of protons but different numbers of neutrons.
Same chemical properties, different mass and stability.
Example: Carbon-12, Carbon-14.
3⃣ Nucleon Number (A)
Total number of protons and neutrons in the nucleus.
4⃣ Atomic Number (Z)
Number of protons in an atom. Determines which element it is.
5⃣ Radioactivity
The spontaneous decay of unstable atomic nuclei, releasing energy and/or particles to reach stability.
6⃣ Alpha Decay (α)
Emits a helium nucleus (24He^4_2He24He).
Nucleus loses 2 protons and 2 neutrons → A–4, Z–2.
Low penetration, stopped by paper.
7⃣ Beta-minus Decay (β⁻)
A neutron changes into a proton, emitting an electron and antineutrino.
Atomic number increases by 1.
Moderate penetration, stopped by aluminium.
8⃣ Gamma Decay (γ)
Release of high-energy electromagnetic radiation from the nucleus.
No change in A or Z — only energy decreases.
Highly penetrating, stopped by thick lead or concrete.
9⃣ Half-Life (T₁/₂)
The time required for half of the radioactive nuclei in a sample to decay.
Constant for each isotope, independent of amount or temperature.
10⃣ Activity (A)
The rate of decay — number of decays per second.
Unit: Becquerel (Bq) = 1 decay per second.
11⃣ Ionisation Power
Ability to knock electrons from atoms.
α = high ionisation, β = medium, γ = low.
12⃣ Penetrating Power
Ability to pass through matter.
α = low, β = moderate, γ = high.
13⃣ Conservation Laws in Decay
Charge, mass number, and energy are always conserved in nuclear equations.
14⃣ Nuclear Fission
A heavy nucleus splits into two smaller nuclei, releasing energy and neutrons.
Example: Uranium-235 in nuclear power plants.
Can form a chain reaction as emitted neutrons trigger more fission.
15⃣ Nuclear Fusion
Two light nuclei combine to form a heavier nucleus, releasing energy.
Example: Hydrogen fuses into Helium in the Sun.
Requires extremely high temperatures and pressures to overcome repulsion.
16⃣ Binding Energy
Energy required to break a nucleus into its individual nucleons.
High binding energy → stable nucleus.
Eb=Δmc2E_b = \Delta m c^2Eb=Δmc2.
17⃣ Mass Defect (Δm)
The difference between the mass of separated nucleons and the actual mass of the nucleus.
This missing mass is converted to binding energy.
18⃣ Chain Reaction
A self-sustaining series of fission events where released neutrons trigger further reactions.
19⃣ Radioisotopes in Medicine
Technetium-99m: used for medical imaging (short half-life).
Cobalt-60: used in cancer radiotherapy (emits gamma rays).
20⃣ Standard Model
Organises fundamental particles into:
6 Quarks: up, down, charm, strange, top, bottom.
6 Leptons: electron, muon, tau + their neutrinos.
Forces:
Strong (gluon), Weak (W/Z bosons), Electromagnetic (photon), Gravity (graviton – theoretical).
Explains how all matter and forces interact at a subatomic level.