The experimental examination applied in the 40th International Physics Olympiad held in Merida, Yucatan, Mexico, is presented. The examination consisted of two parts: (1) based on the measurements of a diffraction pattern produced by a diode laser impinging on a sharp edge of a razor blade, the students were asked to estimate the wavelength of the laser. (2) By using the same experimental setup, the contestants had to measure the birefringence of a mica film. For completeness, we give a brief description of the theoretical test. Finally, we present the distribution of marks and show, on the world map, with different colours, the performance of various countries. (Contains 2 tables and 14 figures.)