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The upper disk and the lower disk have exactly the same objective color, and are in identical gray surroundings; based on context differences, humans MERRY CHRISMUKKA KNITTING PATTERN UGLY CHRISTMAS SWEATER perceive the squares as having different reflectances, and may interpret the colors as different color categories; see checker shadow illusion. Some generalizations of the physics can be drawn, neglecting perceptual effects for now:Light arriving at an opaque surface is either reflected “specularly” , scattered , or absorbed—or some combination of these.Opaque objects that do not reflect specularly have their color determined by which wavelengths of light they scatter strongly. If objects scatter all wavelengths with roughly equal strength, they appear white. If they absorb all wavelengths, they appear black.Opaque objects that specularly reflect light of different wavelengths with different efficiencies look like mirrors tinted with colors determined by those differences. An object that reflects some fraction of impinging light and absorbs the rest may look black but also be faintly reflective; examples are black objects coated with layers of enamel or lacquer.