Any process that results in a change in state or activity of an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of detection of, or exposure to, a period of light or dark of a given length, measured relative to a particular duration known as the 'critical day length'. The critical day length varies between species. [ https://www.worldcat.org/search?q=bn%3A0582015952 https://www.worldcat.org/search?q=bn%3A0697037754 https://www.worldcat.org/search?q=bn%3A0709408862 ]