E.2
Quantum Physics
The photoelectric effect, wave-particle duality, de Broglie wavelength, electron diffraction and Compton scattering. HL only.
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E.2 Quantum Physics
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Key Concepts, Quantum Physics
The Photoelectric Effect
When light shines on a metal surface, electrons are emitted, but only if the frequency of the light exceeds a minimum threshold frequency f₀. Classical wave theory predicted that any frequency should eventually eject electrons if the intensity is high enough, but this is wrong. Einstein explained the effect by proposing that light consists of photons, each with energy E = hf. If a photon's energy exceeds the work function φ of the metal (the minimum energy to free an electron), an electron is immediately emitted. Extra energy becomes kinetic energy: E_k(max) = hf - φ. Increasing intensity increases the number of photoelectrons but not their maximum kinetic energy.
Work Function, Threshold Frequency and Stopping Voltage
The work function φ is the minimum energy needed to release an electron from the metal surface. The threshold frequency f₀ = φ/h is the minimum frequency that can cause emission. Below f₀, no electrons are emitted regardless of intensity. The maximum kinetic energy of emitted electrons is measured using a stopping voltage V_s: the emitted electrons are slowed by a reverse voltage until the fastest just fail to reach the collector, so eV_s = E_k(max) = hf - φ. A graph of E_k(max) vs f is a straight line with gradient h, y-intercept -φ and x-intercept f₀.
Wave-Particle Duality
Light behaves as a wave in interference and diffraction experiments, and as a particle (photon) in the photoelectric effect. This is wave-particle duality. De Broglie extended this idea to matter, proposing that all particles also have wave-like properties. The de Broglie wavelength of a particle is λ = h/p = h/(mv), where p is momentum. Particles with greater momentum have shorter wavelengths. An electron accelerated through potential difference V has kinetic energy ½eV = p²/2m, giving λ = h/√(2meV).
Electron Diffraction
Davisson and Germer fired electrons at a crystal lattice and observed diffraction patterns, confirming the wave nature of matter. A cleaner demonstration fires electrons through a thin graphite film: concentric diffraction rings appear on a fluorescent screen, identical in character to X-ray diffraction patterns. The ring diameter decreases when the accelerating voltage is increased (higher speed, shorter wavelength, less diffraction). This direct evidence for the wave nature of electrons was a landmark confirmation of de Broglie's hypothesis.
Compton Scattering
Compton fired X-ray photons at free electrons and found the scattered photons had a longer wavelength (lower energy) than the incident ones. Classical wave theory predicted no change in wavelength. Compton explained it by treating photons as particles with momentum p = h/λ: when a photon collides with an electron, it transfers some momentum and energy, so its wavelength increases. The Compton wavelength shift is Δλ = (h/m_e c)(1 - cosθ), where θ is the scattering angle. This confirmed that photons carry momentum and behave as discrete particles in collisions.
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Frequently Asked Questions, IB Physics Quantum Physics
What is Quantum Physics in IB Physics? ↓
The photoelectric effect, wave-particle duality, de Broglie wavelength, electron diffraction and Compton scattering. HL only. This topic is part of Theme E (Nuclear & Quantum Physics) in the current IB Physics syllabus.
Is Quantum Physics SL or HL in IB Physics? ↓
Quantum Physics is an HL-only topic. It is not assessed in the SL IB Physics exam.
What equations do I need for IB Physics Quantum Physics? ↓
The key equations for Quantum Physics are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Quantum Physics? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Quantum Physics for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.