Two coloured materials are metamers, or form a metameric pair, when they match in colour, but the light they reflect differs in spectral composition. When this is so the surfaces will differ in colour under different incident lighting. The existence of metamerism is a severe difficulty for theories trying to identify colour of surfaces with the physical nature of reflected light. Interestingly, there is no parallel phenomenon for sound, since sounds composed of different energy levels at different frequencies are heard differently.