Answer: Weber's/Weber-Fechner's law of just noticeable difference
This phychologoical law is used to quantify the perception of change for a given stimulus. The perceived change is proportional/constant ratio to the initial stimulus. Thus, it is the minimum amount of a stimulus intensity change that will produce noticeable variation in sensory experience.
As in the example, it has been shown that this law is not function very well for extremes of stimulation.
