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VICPhysicsQuick questions
Unit 4: How have new ideas and ways of thinking developed our understanding of the physical world?
Quick questions on Matter waves and the de Broglie wavelength: VCE Physics Unit 4
9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is example 1. Electron wavelength?Show answer
An electron is accelerated through 200 V. Find its de Broglie wavelength.
What is example 2. Neutron wavelength?Show answer
A thermal neutron at room temperature has $E_k \approx 0.025$ eV (thermal energy). Mass $m_n = 1.67 \times 10^{-27}$ kg.
What is example 3. Tennis ball?Show answer
50 g tennis ball at 30 m/s. $p = 1.5$ kg m/s, $\lambda = 6.6 \times 10^{-34} / 1.5 = 4.4 \times 10^{-34}$ m. Utterly unobservable.
What is mass in grams?Show answer
Always use kg in $\lambda = h / (m v)$.
What is energy as momentum?Show answer
$E_k$ in eV is not the momentum. Convert $E_k$ to joules, then use $p = \sqrt{2 m E_k}$.
What is photon formula for matter?Show answer
Photons have $E = pc$ (relativistic). Non-relativistic matter has $E = p^2 / (2 m)$. The two relations give different $\lambda$.
What is mixing up the formula?Show answer
$\lambda = h / p$, not $\lambda = h \times p$ or $\lambda = p / h$.
What is forgetting relativistic correction at very high energies?Show answer
For electrons accelerated to MeV energies, the non-relativistic formula breaks down. VCE Physics stays in the non-relativistic regime.
What is confusing matter wave with electromagnetic wave?Show answer
The matter wave is not an electromagnetic wave; it is the quantum probability amplitude. It does not carry energy in the EM sense.