Theme E · Nuclear and Quantum Physics · HL only · 8 hours
E.2 Quantum Physics
How does quantum physics reconcile the wave and particle nature of light and matter?
Higher Level only — no Standard Level content in this topic
The photoelectric effect was the experiment that broke classical physics. When light shines on a metal surface, electrons are ejected — but only if the frequency exceeds a threshold value, regardless of intensity. Classical wave theory predicted that intensity should determine emission; instead, it is frequency that matters. Einstein explained this in 1905: light comes in packets (photons) of energy E = hf, and a photon must have enough energy to release an electron. This is how solar cells work.
De Broglie completed the symmetry: if light waves can behave as particles, perhaps particles can behave as waves. His equation λ = h/p assigns a wavelength to any particle. Electron diffraction experiments confirmed this — electrons diffract through crystal lattices just as X-rays do. Matter and light are both wave-particle dual, and Heisenberg's uncertainty principle sets fundamental limits on what can be simultaneously known about any quantum system.
Light of wavelength 250 nm shines on a sodium surface (work function 2.3 eV). Calculate the maximum kinetic energy of the emitted photoelectrons in eV and in joules. What is the threshold frequency for sodium? (h = 6.63 × 10⁻³⁴ J s; 1 eV = 1.6 × 10⁻¹⁹ J)