Which of the following experimental observations provides direct evidence for the existence of discrete electron energy levels in isolated atoms?
The continuous spectrum of electromagnetic radiation emitted by a blackbody at high temperature.
The emission of photons of only specific, discrete wavelengths from a low-pressure gas in a discharge tube.
The release of electrons from a metal surface only when the incident radiation exceeds a specific threshold frequency.
The diffraction pattern produced when a collimated beam of electrons passes through a thin metal foil.
