Theme E — Nuclear and Quantum Physics
E.2.6 Compton Scattering — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 16 of 45 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
E.2.6 Compton Scattering — IB PHYSICS HL notes: Describe Compton scattering as photon–electron collision and use Δλ = (h/mec)(1 − cos θ).Definition — the interaction of a high-energy photon (X-ray or gamma) with an orbital electron that increases the photon's wavelength and eject…
- Experiment
- Classical prediction
- Significance
- Energy transfer
- Conservation
- Photon properties
Part 1 of 7What Compton scattering is
Definition
- Definition the interaction of a high-energy photon (X-ray or gamma) with an orbital electron that increases the photon's wavelength and ejects the electron
Experiment
- Compton (1923) fired monochromatic X-rays at graphite and measured the scattered wavelength
Classical prediction
- Classical prediction electrons oscillating at the wave frequency would re-radiate the same wavelength, so no shift
Significance
- Significance further evidence for the particle nature of light
Part 2 of 7Collision picture
Energy transfer
- Energy transfer the photon gives some energy to the electron
Photon
- Photon it is deflected, its energy falls and its wavelength increases
Electron
- Electron it is ejected from the atom and recoils on the other side of the scattered photon
Conservation
- Conservation energy and momentum are both conserved
Photon properties
- E = hc/λ and p = h/λ
Part 3 of 7The Compton formula
Equation
- Δλ = λf − λi = (h/mec)(1 − cos θ)
Symbols
- θ is the scattering angle of the photon and me the electron mass
Compton wavelength
- Compton wavelength h/mec = 2.43 × 10−12 m
Assumption
- Assumption the electron is initially at rest
Independence
- Δλ does not depend on the original wavelength
Part 4 of 7Features of the shift
| Scattering angle θ | Δλ |
|---|---|
| 0° | 0 (no change) |
| 60° | half the Compton wavelength (1.2 pm) |
| 90° | h/mec = 2.43 pm |
| 180° | maximum, 2h/mec = 4.86 pm |
Features of the shift
Trend
- Trend the greater the scattering angle, the longer the wavelength
Visible shift
- Visible shift the shift is a noticeable fraction of λ only when λ is small, as for X-rays
Part 5 of 7Energy of the recoil electron
Kinetic energy
- Ke = hc/λ − hc/λ′ = hcΔλ/(λλ′)
Angle dependence
- Angle dependence it increases with the scattering angle
Direction
- Direction find the recoil from momentum conservation
Detection
- Detection recoil electrons can be seen in a cloud chamber
Useful constant
- Useful constant hc = 1.99 × 10−25 J m = 1.24 × 10−6 eV m
Part 6 of 7Compton scattering and the photoelectric effect
| Feature | Photoelectric effect | Compton scattering |
|---|---|---|
| Photon | Completely absorbed | Partly transfers energy and is scattered |
| Electron | Bound in the metal | Essentially free |
| Energy sharing | Work function plus kinetic energy | Photon energy decreases, electron gains kinetic energy |
| Momentum | Taken up by the lattice | Shared between photon and electron |
Part 7 of 7More from the specification
More from the specification
Compton Scattering Mechanism
- Collision between an incident X-ray photon and a loosely bound electron causes the photon to deflect with increased wavelength and reduced energy, proving photon momentum p = h / lambda.
Photoelectric Threshold Frequency
- Photoelectrons are emitted from a metal plate surface only when incident photon frequency exceeds a characteristic threshold frequency f0, regardless of radiation intensity.
Stopping Potential and Kinetic Energy
- Applying an opposing stopping potential Vs halts the fastest emitted photoelectrons,
- where maximum kinetic energy satisfies Ek,max = eV_s = hf - Phi.
De Broglie Wavelength and Diffraction
- Matter particles possess dual wave-particle properties with a de Broglie wavelength lambda = h / p, directly demonstrable through electron diffraction patterns.
Wave-particle duality
- The property that radiation and matter show both wave and particle behaviour.
Intensity
- Power per unit area perpendicular to the wave direction.
Threshold
- The output value at which a switching circuit changes state.
Current
- Flow of electrons in a circuit.
Diffraction
- Spreading of waves through a gap or around an edge
Theory
- A well-tested, wide-ranging explanation supported by evidence.
Threshold frequency
- The minimum frequency that causes photoelectric emission
Photoelectric current
- The charge per second carried by photoelectrons, proportional to the number emitted per second.
Stopping potential
- The p.d. that stops the most energetic photoelectrons
E = hf
What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Key terms
| Term | Meaning |
|---|---|
| Compton scattering | The interaction of a photon with an electron in which the photon's wavelength increases. |
| Compton wavelength | h/mec = 2.43 × 10−12 m. |
| Scattering angle | The angle θ through which the photon is deflected. |
| Recoil electron | The electron ejected with the energy lost by the photon. |
Avoid these mistakes
1
Saying the electron absorbs the whole photon — the scattered photon keeps some energy.
2
Thinking the shift depends on the incident wavelength — it depends only on the angle.
3
Thinking the scattered photon has a shorter wavelength — it has lost energy so λ increases.
4
Forgetting that momentum as well as energy is conserved.
5
Using the Compton formula for visible light and expecting an observable shift.
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