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It depends which kind of noise. http://www.mathworks.com/help/images/ref/imnoise.html
Most of the supported types are essentially randn() * scale + mean
This is all spelled out in the help for the different types of noise you can choose from. It assumes the image is normalized 0-1 and then adds the noise you selected, then it unnormalizes the image to get back to the original intensity range. Was there one particular type of noise that you did not understand the definition of?