Here's the definition from the 'Oxford Companion of the Photograph' of reciprocity law.
"The Reciprocity Law states that photochemical effects depend solely on the light energy absorbed. It underlies the relation between exposure duration and f-number (e.g that 1/30s at f/11 gives the same result as 1/60s at f/8). The law tends to break down at low light levels when the indicated exposure duration exceeds a few seconds. If the law is followed rigidly the result will be underexposure, and possibly poor colour balance. Increasing the exposure duration corrects the underexposure, but often with increased contrast. Film manufacturers sometimes provide tables for exposure compensation and correction filters."
This from Wikipedia on reciprocity
Reciprocity (photography - Wikipedia, the free encyclopedia)
Hope that helps Sian