Imagine yourself in a dark classroom reading powerpoint slides. if an audience member were to check the internet using her cell phone and causing her screen to light up, chances are that many people would notice the change in illumination in the classroom. however, if the same thing happened in a brightly lit classroom during a discussion, very few people would notice. the cell phone brightness does not change, but its ability to be detected as a change in illumination varies dramatically between the two contexts. this is an example of

Weber's Law

Explanation:

Weber-Fechner's Law relates the physical magnitude of a stimulus to the subjective intensity of a person's sensation. It goes for any sensory perception, be it auditory, visual, thermal, tactile, taste or olfactory. Generally speaking, the Weber-Fechner Law can be stated as follows: the increase in stimulus necessary to produce the minimum increase in sensation is proportional to the pre-existing stimulus. Cell phone light is a stimulus, which is increased in low light situations, so this light is more noticeable in a dark and quiet room than in a room with lots of people talking.