  This problem was given on the Open Experimental Physics Olympiad (OEPO) "The
day of the photon" on 25 April 2015. The Olympiad was a part of the celebration
of 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.html
and was organized by the Sofia Branch of the Union of Physicist in Bulgaria and
the Regional Society of Physicists of Strumica, Macedonia.
