Purple needs no wavelength. A dress can look purple because color is a visual interpretation of light, not a requirement that every perceived shade occupy its own address on the spectrum. The distinction matters here. Unlike violet, which can be produced by a narrow band of short wavelengths, purple generally arises from mixtures that stimulate vision differently.
The fabric supplies ingredients. Under white illumination, its dyes absorb some wavelengths and reflect others, often returning a mixture weighted toward the short and long ends of the visible spectrum. The eye compares responses. Three classes of cone cells have overlapping sensitivities to light, so their relative activity, rather than any individual photon's identity, supplies the information used to distinguish colors. Purple is a relationship. When short- and long-wavelength-sensitive cones respond sufficiently relative to medium-wavelength-sensitive cones, later neural processing can produce the experience called purple, although the exact shade depends on the full pattern.
The brain keeps accounts. Through opponent processing, neural circuits compare cone signals along channels associated with red versus green and blue versus yellow, while surrounding colors and adaptation also influence the result. Names muddy the distinction. Everyday speech often treats purple and violet as interchangeable, but spectral violet and nonspectral purples do not have the same physical explanation, even when they look similar. Illumination gets a vote. Change the lamp, and the fabric receives a different mixture to reflect, potentially changing its appearance without changing its dyes. The dress hangs still; its purple belongs to an encounter.
