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.
