Stern–Gerlach experiment demonstrated that the spatial orientation of angular momentum is quantized.
Illustration Diagram of Stern–Gerlach Experiment
Reference Source:
[1] Wikipedia Contributors. “Stern–Gerlach Experiment.” Wikipedia, Wikimedia Foundation, 12 May 2021, en.wikipedia.org/wiki/Stern-Gerlach_experiment. Accessed 11 Sept. 2021.
[2] Butler, David. “Classroom Aid – Electron Spin and the Stern-Gerlach Experiment.” YouTube, 5 May 2021, www.youtube.com/watch?v=lYl6uLNT9vc&t=78s. Accessed 11 Sept. 2021.
[3] Eduphile. “Stern Gerlach Experiment.” YouTube, 22 Jan. 2021, www.youtube.com/watch?v=oJkVqnaidxo. Accessed 11 Sept. 2021.
Also, read this article about Photoelectric Effect.