My apologies to those of you hoping to read about physics gone mad; the truth is that psychophysics is actually the study of our sensitivity to stimuli of different strengths. This is generally done by attempting to measure different types of thresholds.
Absolute threshold refers to the minimum amount of stimulation necessary for a stimulus to be detected. For example, the absolute threshold for vision would be the dimmest light that we could possibly see or for hearing it would be the softest tone that we’re able to hear.
Measuring absolute threshold is difficult because it’s often hard to know if a participant is actually detecting a stimulus or just guessing. As we approach the minimal intensity, even the participant will have a hard time knowing if he really saw or heard something. In order to accommodate for this, psychophysicists (who may or may not be mad scientists) generally consider the absolute threshold to be the point at which accuracy for detecting a stimulus reaches 50% and higher. Once a participant is able to achieve 50% accuracy, it can be assumed that the participant is actually detecting the stimulus.
In addition to knowing about the minimum intensity necessary for perception, psychophysics also investigates how well we can differentiate between two stimuli that are both above the absolute threshold. In other words, how much change is necessary (in the brightness of a light, the volume of a sound, etc.) for us to detect that a change has occurred? While many psychological concepts have wonderfully creative names, this is not one of them. The minimum amount of change that we can actually detect is referred to as the just-noticeable difference (JND).
In attempting to measure just-noticeable differences, we run into the following question: Why is it that people can tell the difference between the brightness of 1 versus 2 candles, but not the difference between 100 and 101 candles? Similarly, when I’m at the gym, I can easily feel the difference between a 1lb weight and a 2lb weight (I’m really hitting those biceps hard), but I can’t detect the difference between 100lbs and 101lbs. The reason for this is that just-noticeable differences aren’t constant amounts of change, they are constant proportions of change. This concept is known as Weber’s Law (Ernst Weber was German so it’s pronounced Vay-ber, not Web-er).
For example, in the case of detecting changes in weight, people generally need a change of about 2% in order to detect the difference. This explains why an envelope with 2 sheets of paper inside feels noticeably heavier than an envelope with only one sheet, but why a 500 page textbook feels the same as a textbook with 505 pages.
This post is an excerpt from Master Introductory Psychology: Compete Edition
