Which of the following observations provides direct evidence that electrons within gas atoms occupy discrete energy levels?
The emission of a continuous spectrum of light when a solid tungsten filament is heated.
The emission of a line spectrum containing specific wavelengths from a low-pressure gas discharge tube.
The ejection of electrons from a metal surface only when the incident radiation exceeds a threshold frequency.
The diffraction of electrons into concentric rings when passed through a thin sheet of polycrystalline graphite.
31 exam-style questions on AQA A Level Physics Electromagnetic radiation and quantum phenomena, covering The photoelectric effect, Collisions of electrons with atoms, Energy levels and photon emission, and Wave-particle duality. Each one has a worked solution and a mark scheme showing where the marks go.