HSC Physics — Module 7
de Broglie Wavelength — Flashcards & Quiz
Louis de Broglie proposed that every particle with momentum has an associated wavelength λ = h/p, extending wave-particle duality from photons to matter. HSC Physics Module 7 tests your ability to calculate de Broglie wavelengths for electrons and atomic nuclei, explain why macroscopic objects have negligible wavelengths, and link the concept to experimental evidence like the Davisson-Germer electron diffraction experiment and quantised orbits in the Bohr model.
Key Points
- Every particle with momentum p has an associated wavelength λ = h/p = h/(mv), where h is Planck's constant.
- For macroscopic objects (a cricket ball, a person), the de Broglie wavelength is ~10⁻³⁴ m — far too small to observe wave behaviour.
- For electrons (small mass, comparable momentum), the wavelength is ~10⁻¹⁰ m, similar to atomic spacings in crystals — enabling electron diffraction.
- Davisson-Germer experiment (1927) observed electron diffraction from a nickel crystal, confirming the wave nature of matter predicted by de Broglie.
- In Bohr's model, quantised orbits correspond to standing-wave conditions where an integer number of de Broglie wavelengths fit around the circumference: 2πr = nλ.
- Wave-particle duality: every quantum object is both a wave and a particle — which behaviour you observe depends on the experiment you perform.
Common Mistakes to Avoid
- Applying λ = h/p to macroscopic objects without noting their wavelength is negligibly small.
- Confusing momentum (p = mv) with kinetic energy (½mv²).
- Missing the connection between de Broglie wavelengths and electron diffraction.
- Forgetting that de Broglie's hypothesis extended wave-particle duality from photons to matter.
- Claiming atoms diffract — they can, but the wavelengths are extremely short.
Exam Strategy
HSC Module 7 de Broglie questions ask you to calculate wavelengths for electrons or particles and link to experimental evidence. Method: (1) calculate p = mv (or from KE via p = √(2mE)), (2) apply λ = h/p, (3) compare to typical atomic spacings (~10⁻¹⁰ m) to justify diffraction.
Sample Flashcards
Q1: What is the de Broglie wavelength?
Louis de Broglie proposed that all matter has wave-like properties. The de Broglie wavelength: λ = h/p = h/(mv), where h is Planck's constant, p is momentum. Larger objects have negligibly small wavelengths; the effect is only significant for subatomic particles.
Sample Quiz Questions
Q1: A fast-moving electron has a shorter wavelength than a slow-moving electron.
Answer: TRUE
λ = h/(mv). As v increases, momentum increases, and wavelength decreases.
Q2: A macroscopic object like a cricket ball has a measurable de Broglie wavelength.
Answer: FALSE
While technically having a de Broglie wavelength, for a 0.15 kg ball at 30 m/s, λ ≈ 1.5 × 10⁻³⁴ m — far too small to measure or detect.
Revision Tip
de Broglie calculations are short and formulaic — drill Revizi flashcards with 5-6 different particles (electron, proton, neutron, alpha particle).
Related Concepts
Last updated: March 2026 · 1 flashcards · 2 quiz questions