Problem of the 8-th Experimental Physics Olympiad, Skopje, 8 May 2021 Determination of Planck Constant by LED

This is the problem of 8$^\mathrm{th}$ International Experimental Physics Olympiad (EPO). The task EPO8 to determine Plank constant $\hbar=h/2\pi$ using given set-up with LED. If you have an idea how do it, it and send us result; skip reading detailed step by instructions increasing difficulties. We expect participants follow suggested items -- they are instructive for physics education in general.Only historical remarks first section can be omitted during without loss generality. Participants should try solving as much tasks paying attention age categories: give your best.