Weber's Law is the psychophysics principle that the just noticeable difference (JND) between two stimuli is a constant proportion of the original stimulus intensity, meaning you detect changes in percentages, not fixed amounts (Topic 3.1, Principles of Sensation).
Weber's Law describes how much a stimulus has to change before you actually notice the change. The key insight is that the just noticeable difference (JND) isn't a fixed amount. It's a constant proportion of the original stimulus. Add one candle to a dark room and you'll see the difference instantly. Add one candle to a stadium full of floodlights and you'll notice nothing, because one candle is a tiny fraction of the total light.
In other words, your senses work in percentages. If you can tell the difference between a 100-gram weight and a 102-gram weight (a 2% change), then for a 1,000-gram weight you'd need about 20 extra grams before you'd notice. The proportion (often written as ΔI/I = k, the Weber fraction) stays roughly constant for a given sense, even though the raw amount of change needed keeps growing as the stimulus gets stronger. This is one of the core findings of psychophysics, the field that studies how physical stimuli map onto psychological experience.
Weber's Law sits in Topic 3.1, Principles of Sensation, alongside absolute threshold, difference threshold, and sensory adaptation. It's the quantitative backbone of how the CED frames sensation. Thresholds aren't fixed gates; they scale with the stimulus. Knowing Weber's Law lets you answer the classic exam scenario where a person notices a small change in a weak stimulus but misses a much bigger change in a strong one. It also sets up the later psychophysics laws (Fechner's Law and Stevens' Power Law), which build on the same proportional logic to describe the whole stimulus-to-sensation relationship.
Keep studying AP Psychology Unit 3
Absolute Threshold (Unit 3)
The absolute threshold is the minimum stimulation you can detect 50% of the time, so it's about detecting that something is there at all. Weber's Law is about the difference threshold, detecting that something changed. AP questions love swapping these two, so keep them straight.
Fechner's Law (Unit 3)
Fechner took Weber's constant proportion and stacked it into a full equation. Because each JND requires a proportionally bigger jump in intensity, sensation grows logarithmically. Fechner's Law is basically Weber's Law extended across the entire range of a stimulus.
Stevens' Power Law (Unit 3)
Stevens found that for some senses, like pain, sensation grows faster than Fechner's logarithm predicts. His power law is the modern refinement, but it still starts from Weber's core idea that perception scales with proportion, not raw amount.
Psychophysics (Unit 3)
Weber's Law is one of the founding results of psychophysics, the field that measures how physical energy (light, sound, weight) translates into psychological experience. If a question asks for an example of psychophysics research, Weber's weight-comparison experiments are a textbook answer.
Weber's Law shows up almost entirely in multiple-choice scenario questions. A typical stem gives you a real-world situation, like someone noticing one extra candle in a dim room but not in a bright one, or detecting a small price increase on a cheap item but not an expensive one, and asks which principle explains it. Your job is to recognize the proportional pattern and pick Weber's Law over absolute threshold, sensory adaptation, or signal detection theory. Practice questions also push on its limits, like what ethical issues arise if psychologists ignore individual differences when applying Weber's Law (people's Weber fractions vary, so treating one constant as universal can misrepresent some groups). No released FRQ has used the term verbatim, but it fits FRQ prompts that ask you to apply sensation concepts to a scenario.
The absolute threshold answers "can you detect this stimulus at all?" Weber's Law answers "can you detect a change in a stimulus you already sense?" If the question involves the weakest detectable stimulus (the faintest sound, the dimmest light), that's absolute threshold. If it involves comparing two stimuli or noticing a difference, that's the difference threshold, and Weber's Law tells you the difference must be a constant proportion of the original.
Weber's Law states that the just noticeable difference (JND) is a constant proportion of the original stimulus intensity, written as ΔI/I = k.
You notice changes in percentages, not fixed amounts, so a stronger stimulus needs a bigger absolute change before you detect any difference.
Weber's Law applies to the difference threshold, not the absolute threshold, which is about detecting a stimulus at all.
The Weber fraction differs across senses, and Fechner's Law and Stevens' Power Law later built on Weber's proportional idea.
On the exam, spot Weber's Law in scenarios where someone notices a small change in a weak stimulus but misses a larger change in a strong one.
Weber's Law is the principle that the just noticeable difference between two stimuli is a constant proportion of the original stimulus, not a fixed amount. It's part of Topic 3.1, Principles of Sensation, and explains why one candle is obvious in a dark room but invisible in a bright one.
No. The absolute threshold is the minimum stimulus you can detect 50% of the time, while Weber's Law describes the difference threshold, how much a stimulus must change before you notice. Detection of presence versus detection of change is the distinction exam questions test.
ΔI/I = k, where ΔI is the smallest detectable change, I is the original stimulus intensity, and k is the Weber fraction, a constant for a given sense. So if k is 0.02 for weight, you need about a 2% change to notice, whether the weight is 100 grams or 1,000 grams.
Weber's Law describes one comparison, the proportion needed to notice a single change. Fechner's Law stacks those proportional steps into a full equation, concluding that sensation grows logarithmically as stimulus intensity increases. Stevens' Power Law later refined Fechner's version for senses like pain.
No. The Weber fraction varies across senses (vision, hearing, and weight each have different constants) and across individuals. Practice questions even raise the ethical problem of applying one constant to everyone without accounting for individual differences.