Final answer:
Weber's fraction is a measure of the just-noticeable difference in stimulus intensity relative to the original stimulus magnitude, demonstrating that the JND is proportional according to Weber's Law.
Step-by-step explanation:
Weber's fraction expresses the just-noticeable difference (JND) as a proportion of the original stimulus. Weber's Law implies that for any given sense, the perceived change in a stimulus will be proportional to the original intensity of the stimulus. For example, if a bag of rice weighs one pound, and a person can just notice a weight increase when it's 1.1 pounds, then the Weber's fraction would be 0.1/1 (or 10%). This concept helps to explain the ratio of intensities between two sounds of identical frequency where one is just barely discernible as louder than the other. To adhere to Weber's Law, these intensities must be in a consistent ratio that correlates with the baseline intensity, emphasizing that the JND is a proportional quantity.