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Theme E — Nuclear and Quantum Physics

E.2.3 The Particle Nature of Light — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 45 1 quiz

📚 IB Physics HL Hub / Topical Revision
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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.
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Lesson summary

E.2.3 The Particle Nature of Light — IB PHYSICS HL notes: Describe evidence for photons, the gold-leaf electroscope, photon momentum p = h/λ and the force exerted by light.Quanta — light is emitted, travels and is absorbed in discrete bundles called photons, E = hfWhat the symbol…

  • What the symbols mean
  • Properties
  • Quantised energy
  • Observation 1
  • Observation 2
  • Observation 3

Part 1 of 7Photons

Quanta
  • Quanta light is emitted, travels and is absorbed in discrete bundles called photons, E = hf
    What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Properties
  • Properties photons are neutral, have no rest mass and always travel at c in a vacuum
Duality
  • Duality light also shows wave behaviour such as interference and diffraction
Quantised energy
  • Quantised energy electromagnetic waves carry energy in discrete packets

Part 2 of 7Gold-leaf electroscope

1Set-up
  • Set-up a zinc plate on a rod with a gold leaf, given a negative charge so the leaf is repelled
2Process
  • UV light releases photoelectrons, the plate loses charge and the leaf falls
3Observation 1
  • Observation 1 the leaf falls instantly in UV light
  • UV photons carry enough energy to free electrons
4Observation 2
  • Observation 2 the leaf falls slower at lower UV intensity, but still instantly
  • Intensity sets the rate of emission and the instant fall supports one-to-one photon transfer
5Observation 3
  • Observation 3 no fall at lower frequencies such as visible light
  • Photons then have too little energy
6Observation 4
  • Observation 4 no fall at low frequency even with greater intensity
  • More photons cannot help if each has too little energy

Part 3 of 7Why the wave model fails

Wave prediction
  • Wave prediction electrons would gain energy with each wavefront, so any frequency should work given time
Intensity
  • Intensity wave energy depends on intensity, so kinetic energy should depend on intensity
Photon model
  • Photon model one photon interacts with one electron and the photon energy depends on frequency
Conclusion
  • Conclusion the observations support the particle nature of light

Part 4 of 7Photon momentum

Equation
  • Equation p = E/c = hf/c = h/λ
Unit
  • Unit kg m s−1
Value
  • Value a 500 nm photon has p = 1.3 × 10−27 kg m s−1
Effect
  • Effect photons that are absorbed or reflected transfer momentum, exerting a force

Part 5 of 7Force from light

Absorbing surface
  • F = P/c for a beam of power P, since the photon rate is P/E and each photon delivers E/c
Perfect reflection
  • F = 2P/c at normal incidence because each photon's momentum changes from +p to −p

Part 6 of 7Evidence for the particle model

Evidence for the particle model
Photoelectric effect
  • Photoelectric effect threshold frequency, instant emission and Ek,max independent of intensity
Line spectra
  • Line spectra photons have energies equal to differences between atomic energy levels
Compton scattering
  • Compton scattering photons behave like particles colliding with electrons
Detectors
  • Detectors dim light registers as individual photon events

Part 7 of 7More from the specification

More from the specification
Photon energy and Planck's constant
  • Light behaves as discrete quanta of energy termed photons, where photon energy is directly proportional to frequency: E = hf = hc / lambda.
Einstein's photoelectric equation
  • In the photoelectric effect, absorbed photon energy overcomes metal work function (Phi) to release photoelectrons: hf = Phi + Ek,max.
De Broglie matter wavelength
  • Moving particles exhibit wave-particle duality with a de Broglie wavelength inversely proportional to momentum: lambda = h / p.
Wavelength
  • Distance between a point on one wave and the same point on the next wave
Current
  • Flow of electrons in a circuit.
Radiation
  • Transfer by electromagnetic waves, requiring no medium
Theory
  • A well-tested, wide-ranging explanation supported by evidence.
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
Scattering angle
  • The angle θ through which the photon is deflected.
Wave-particle duality
  • The property that radiation and matter show both wave and particle behaviour.

Key terms

TermMeaning
PhotonA quantum of electromagnetic energy, E = hf, with no rest mass.
Photon momentump = h/λ.
Gold-leaf electroscopeA device in which a charged gold leaf falls when photoelectrons discharge the plate.
Radiation forceThe force on a surface from photon momentum transfer, P/c for absorption.

Avoid these mistakes

1
Saying photons have momentum because they have mass — they have no rest mass and p = h/λ.
2
Confusing the factor of 2 for reflection compared with absorption.
3
Thinking the particle model replaces the wave model — both are needed.
4
Using E = ½mv2 for a photon — E = hf.
5
Forgetting to convert nm to m in p = h/λ.

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