Red
Red is a color corresponding to electromagnetic radiation with dominant wavelengths approximately between 620 and 750 nanometers in the visible spectrum, evoking a distinct perceptual response in human vision via stimulation of long-wavelength-sensitive cones in the retina.[1][2] In the additive color model employed in digital displays and lighting, red functions as one of three primary colors alongside green and blue, where their combinations produce the full gamut of visible hues through varying intensities of light emission.[3][4] Physically, red's appearance in natural phenomena arises from phenomena such as Rayleigh scattering in sunsets, iron oxide on planetary surfaces like Mars, and hemoglobin in blood, reflecting causal mechanisms rooted in molecular absorption and reflection of specific wavelengths.[5] Historically, red pigments, primarily iron oxide-based ochres, represent the earliest known colorants used by humans, with evidence of application in Paleolithic cave art dating back tens of thousands of years, predating more complex synthetic variants like vermilion derived from cinnabar.[6][7] These attributes underscore red's foundational role in optics, biology, and material science, distinct from subtractive pigment models where it approximates a secondary mixture.[8]Physical Properties
Definition and Spectrum
Red is the color associated with the long-wavelength portion of the visible electromagnetic spectrum, specifically wavelengths ranging from approximately 620 to 750 nanometers.[9] [10] [11] This positions red adjacent to orange at the shorter end (around 590–620 nm) and bordering the near-infrared region beyond 750 nm, which exceeds human visual detection.[12] The upper limit near 750 nm reflects decreased retinal sensitivity, as photon energy lowers and overlaps with non-visible thermal radiation.[9] Within this spectrum, red encompasses a continuum of hues rather than a single wavelength, with perceptual distinctions arising from differential stimulation of retinal cone cells. Light at 620–650 nm evokes orangish-red tones due to partial medium-wavelength (M-cone) activation, while 650–700 nm produces purer reds dominated by long-wavelength (L-cone) response, and wavelengths approaching 750 nm yield darker, less saturated perceptions.[13] /26%3A_Vision_and_Optical_Instruments/26.03%3A_Color_and_Color_Vision) Boundaries are conventional and lack sharp physical demarcation, as the spectrum is continuous; color categorization depends on human physiology and context, with variations across observers due to factors like age-related lens yellowing or genetic anomalies in cone pigments.[9] In additive color models like RGB, red serves as a primary, mixing with green and blue to span perceivable colors, but spectral red remains distinct as monochromatic long-wave light.[14]Light Wavelength and Reflection
Red light corresponds to electromagnetic waves in the visible spectrum with wavelengths ranging from approximately 620 to 750 nanometers.[15][16] This positions red at the longer-wavelength extreme of human-visible light, adjacent to infrared radiation beyond 750 nm.[5] The exact perceptual boundaries can vary slightly due to individual differences in retinal sensitivity, but empirical measurements from spectroscopy consistently place pure red hues within this band.[17] Objects appear red under white illumination because their surfaces selectively reflect photons in the 620–750 nm range while absorbing shorter wavelengths such as blue and green.[18][19] This selective reflection arises from the atomic and molecular structure of the material: electrons in surface atoms or pigments absorb energy from non-red photons, exciting to higher orbitals and re-emitting minimal reflected intensity at those wavelengths, whereas red photons interact less strongly and are scattered back toward the observer.[20][21] For instance, a ripe tomato reflects predominantly red light due to carotenoids like lycopene, which have absorption peaks in the violet-blue region (around 400–500 nm), leaving red wavelengths relatively unabsorbed.[22] In diffuse reflection, common for matte red surfaces, incident light scatters in multiple directions without preserving image-forming coherence, yet the spectral selectivity preserves the red hue as long as the surface roughness scale exceeds the wavelength (typically >750 nm for visible red).[23] Specular reflection on polished red materials, such as metallic paints, mirrors the wavelength selectivity but with coherent wavefronts, enhancing gloss while maintaining color fidelity under varying angles of incidence per the law of reflection (angle of incidence equals angle of reflection).[24] This process is wavelength-independent in ideal mirrors but material-specific in pigmented coatings, where dielectric interfaces or thin-film interference can fine-tune reflected red intensity.[25] Empirical verification comes from spectrophotometry, which measures high reflectance (>50%) in the red band for such surfaces under broadband illumination.[19]Pigments and Chemical Composition
Red pigments consist of inorganic and organic compounds that absorb light in the blue-green spectrum (approximately 450-550 nm) while reflecting longer red wavelengths (620-740 nm), resulting from electronic transitions or charge transfer within their molecular or crystalline structures. Natural mineral-based red pigments, such as hematite (α-Fe₂O₃), derive their color from d-d electron transitions in iron(III) ions, producing earthy reds used in Paleolithic cave paintings dating back at least 40,000 years.[26] Red ochre, a natural mixture of hematite and clay, provided the first widespread red pigment for human art, with archaeological evidence from sites like Lascaux confirming its prehistoric application.[27] Vermilion, sourced from cinnabar (HgS), exhibits a brilliant scarlet hue due to its semiconductor bandgap allowing selective reflection of red light; this mercury(II) sulfide was mined and processed for use in ancient Roman murals from the 1st century BCE and in Chinese lacquerware starting in the Song dynasty (960-1279 CE).[7] Red lead, or minium (Pb₃O₄), a lead tetroxide produced by roasting lead(II) oxide at high temperatures since ancient Greek times around 300 BCE, yields a warm orange-red tone from lead-oxygen charge transfer complexes and was favored for medieval manuscript illumination due to its opacity and adhesion.[7] Organic natural red pigments include alizarin (C₁₄H₈O₄), extracted from madder roots (Rubia tinctorum), where the anthraquinone structure enables π-π* transitions absorbing violet light; this dye dominated textile and art applications from antiquity through the 19th century until synthetic alternatives displaced it.[28] Carmine, derived from carminic acid in cochineal insects, offered a translucent crimson but required mordants like alum for fixation on fabrics.[28] Synthetic red pigments emerged in the 18th century with Mars reds, calcined synthetic iron oxides (Fe₂O₃ variants) mimicking natural ochres but with purer, more consistent particle sizes for improved durability.[29] Alizarin was first synthesized in 1868 by German chemists Carl Graebe and Carl Liebermann via oxidation of anthracene, replicating madder dye at lower cost and higher yield, effectively ending large-scale natural madder cultivation.[30] Cadmium red, a cadmium sulfoselenide (Cd(SₓSe₁₋ₓ)) developed in the early 20th century through high-temperature calcination of cadmium sulfide and selenium mixtures, provides opaque, lightfast reds resistant to fading, though its toxicity limits modern use.[31] Contemporary synthetics like diketopyrrolopyrrole (DPP) reds, such as Pigment Red 254 (C₁₈H₁₀Cl₂N₂O₂), achieve high chroma and stability through conjugated heterocyclic systems.[32]