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This problem was given on the Open Experimental Physics Olympiad (OEPO) "Theday of the photon" on 25 April 2015. The Olympiad was a part of the celebrationof the International Year of Light (IYL)http://www.light2015.org/Home/Event-Programme/2015/Competition/Bulgaria-Second-Experimental-Physics-Olympiad--25-April-2015-in-Sofia.-The-Day-of-the-Photon-in-the-International-Year-of-the-Light.htmland was organized by the Sofia Branch of the Union of Physicist in Bulgaria andthe Regional Society of Physicists of Strumica, Macedonia.

Measuring Plank constant with colour LEDs and compact disk