Optics
Color and Light
Wavelengths of visible light interpreted by our eyes — and how objects appear colored
Color is the perception of different wavelengths of visible light (380-750 nm). Objects appear colored because they selectively absorb/reflect/transmit different wavelengths. Three types of cone cells in the eye (red, green, blue) enable color vision. Pure spectral colors (rainbow) vs perceived (paints, screens) follow different mixing rules. Critical for displays, photography, and biology.
- Visible range~380-750 nm
- Three conesS (blue ~420 nm), M (green ~530), L (red ~560)
- Spectral colorsPure wavelengths from rainbow
- Mixing rulesAdditive (light, RGB) — paints subtractive (CMYK)
- Color visionTrichromatic in humans; varies in other animals
- Color blindness~8% of men, <1% of women (genetic)
Interactive visualization
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Visible spectrum
| Color | Wavelength (nm) | Frequency (THz) |
|---|---|---|
| Violet | 380-450 | 668-789 |
| Blue | 450-485 | 618-668 |
| Cyan | 485-500 | 600-618 |
| Green | 500-565 | 531-600 |
| Yellow | 565-590 | 508-531 |
| Orange | 590-625 | 480-508 |
| Red | 625-750 | 400-480 |
Human cone sensitivities
| Cone type | Peak wavelength | Range |
|---|---|---|
| S (short, "blue") | ~420 nm | 360-490 |
| M (medium, "green") | ~534 nm | 440-580 |
| L (long, "red") | ~564 nm | 490-650 |
L and M peaks are surprisingly close (~30 nm apart) — that's why red-green colorblindness is most common.
Color mixing models
| Type | Primary colors | Use |
|---|---|---|
| Additive (light) | RGB (red, green, blue) | Displays, projectors |
| Subtractive (pigments) | CMYK (cyan, magenta, yellow, key/black) | Printing |
| HSL/HSV | Hue, Saturation, Lightness/Value | Computer graphics |
| CIE 1931 xy | Chromaticity coordinates | Standards, science |
| LAB | Lightness, A (green-red), B (blue-yellow) | Perceptual uniform |
Where color matters
- Displays. RGB pixels for TV, monitor, phone. Different display types have different color gamuts.
- Printing. CMYK process; managing color across devices.
- Photography. White balance, color grading, raw processing.
- Lighting design. Color temperature, CRI (color rendering index), match to environment.
- Material science. Pigments, dyes, structural colors (peacocks, butterflies).
- Astronomy. Color photometry of stars (B-V index) reveals temperature, redshift.
- Biology. Camouflage, signaling, pollination by color-sensitive animals.
Common mistakes
- Confusing additive and subtractive. Light mixing → RGB; paint mixing → CMYK. Different rules, opposite logics.
- Thinking colors are absolute. Color depends on illumination, surroundings, observer. Same object looks different in sunlight vs incandescent light.
- Treating color as fundamental physics. Color is PERCEPTION. Light has wavelengths; "color" is what brain interprets. "Blue" doesn't exist outside observer.
- Forgetting trichromatic limitation. Humans only sample 3 wavelength bands. Many colors are "metamers" — different spectra appearing identical (e.g., spectral yellow = red + green light).
- Equating purity with intensity. Saturation (purity) and brightness are independent. Pastel colors = low saturation. Pure neon = high saturation.
- Thinking all animals see similarly. Mantis shrimp, bees, birds — different visual systems. Many see UV; mammals mostly dichromatic.
Frequently asked questions
How do we see different colors?
Three types of cone cells in retina respond to different wavelength ranges. Red (L cones, peak ~560 nm), green (M, ~530), blue (S, ~420). Brain combines signals to perceive colors. Different combinations of red/green/blue activation give all colors. Computer displays exploit this — RGB pixels match cone activation.
What's the difference between additive and subtractive mixing?
Additive (light) — adding wavelengths gives lighter colors. RGB combines: red + green = yellow; all three = white. Used in displays, light shows. Subtractive (paints, inks) — pigments ABSORB wavelengths. CMYK: cyan + magenta + yellow ≈ black. Same color from different methods (e.g., yellow paint absorbs blue, reflects red+green = yellow).
Why is the sky blue?
Rayleigh scattering. Air molecules scatter shorter wavelengths (blue) more strongly than longer (red). Sunlight passing through atmosphere → blue scattered everywhere → sky blue. At sunset, sun's light passes through more atmosphere → most blue scattered out → red/orange remains. Same physics.
Why is grass green?
Chlorophyll absorbs red (~680 nm) and blue (~430 nm) wavelengths for photosynthesis; reflects green. Different plants have different pigments — autumn leaves change color when chlorophyll breaks down, revealing other pigments (carotenoids = orange/yellow, anthocyanins = red).
How do animals see colors differently?
Birds have 4 cone types (UV-sensitive); flowers reveal patterns invisible to humans. Mantis shrimp have 12-16 photoreceptor types. Bees see UV. Dogs are dichromats (red-green colorblind by human standards). Most mammals have only 2 cones; primates re-evolved trichromacy. Octopi may sense color through skin despite being colorblind.
What's color blindness?
Genetic difference in cone cells. Most common — red-green confusion (8% of men, <1% women — X-linked). Total color blindness (rod monochromat) — extremely rare. Tetrachromacy — ~1% of women have 4-cone variant; can distinguish more colors than typical. Diagnosed via Ishihara plates and other tests.
How is color quantified?
CIE 1931 chromaticity diagram. Each color has chromaticity coordinates (x, y). Color space — CIE LAB, RGB, sRGB, Adobe RGB, etc. Different spaces have different gamuts (range of representable colors). Color management critical in printing, displays, photography.