How much brighter must a light get before you can tell? Not a fixed amount — a fixed fraction of what's already there. Slide the background level, drag the test patch, switch the sense — and find the razor's edge where a difference becomes noticeable.
Weber's law: the just-noticeable difference is a constant fraction of the baseline — ΔI/I = k. Slide the background I and drag the test patch's Δ; the JND threshold k·I and the verdict (is Δ ≥ k·I?) recompute exactly. Switching the sense loads that modality's typical Weber fraction (brightness ≈ 8%, lifted weight ≈ 2%, loudness ≈ 5%, taste ≈ 8%) — so the same absolute Δ is easy to notice on a dim background and invisible on a bright one. Real psychophysics (Weber–Fechner).
The Weber fractions are typical textbook values, not your eyes — real thresholds vary with the observer, the exact stimulus, and break down at the extremes of intensity (Weber's law holds over the middle range, not the very faint or very intense). The law ΔI/I = k and every number derived from it are computed exactly; which fraction a given sense truly has is the empirical part.