Colors are the smiles of Nature Leigh Hunt

Introduction Color. Or is it? How beautiful the world is because of color! So easy to understand. It’s one of the first concepts children learn. It’s real. It’s universal. Or is it? It’s not simple. It’s not real. It’s not universal.

Color wheels for mixing paint.

But why should a color wheel work? The spectrum is linear! 575 nm 400 nm 450 nm 520 nm 700 nm Newton’s “color wheel”

Color wheels for mixing paint to neutralize color: OK. yellow green red cyan Not OK for assigning visual complement in spectroscopic analysis of compounds !! magenta blue

cosmic gamma X-rays UV Vis IR Radio induction power Color is our brain’s response to light. “Light” is a term that refers to a electromagnetic radiation. And electromagnetic radiation are waves of different energies that extend over a broad range: frequency, Hz 10 26 10 24 10 22 10 20 10 18 10 16 10 14 10 12 10 10 10 8 10 6 10 4 10 2 Hz cosmic gamma X-rays UV Vis IR Radio induction power wavelength, nm 10 -8 10 -6 10 -4 10 -2 1 10 2 10 4 10 6 10 8 10 10 10 12 10 14 10 16 nm If the electromagnetic spectrum were a piano keyboard….. the visible spectral region would be just one key!

Each of the “colors” of visible light has a corresponding wavelength between 400 to 700 nanometers (or nm). 565 nm Light having a wavelength of near 565 nm will look yellow to most people. 500 600 700 750 650 550 450 400 Is this because the eye has a specific detector (or receptor) for 565 nm yellow light? And a different receptor for 450 nm blue light? And another one for 650 red-orange light? And so on, for every wavelength between 400 and 700 nm? Seems like that would be a LOT of different types of receptors. Nature is “smarter” —and more efficient—than that. Only three different receptors for visible light are used.

Beauty in the eye of the beholder.1 Your eyes have only three color receptors (detectors) - the RGB cones (red-green-blue) It is the brain that interprets visible light as “having” color.

Beauty in the eye of the beholder.2 Yellow light is perceived by our eyes when two color receptors, Red and Green, are stimulated simultaneously. This is indicated on the diagram by the red and green arrows. It is the brain that interprets yellow light as “having” a yellow color.

stimulate the Red and Green receptors. See?” Beauty in the eye of the beholder.3 “What about red-orange?”, you say. “Red-orange light would also simultaneously stimulate the Red and Green receptors. See?” Aaahhh…..true. But look! The relative lengths of the arrows are different. The green arrow, i.e. G-receptor, is much less stimulated than the R -receptor. So, red-orange light is seen as different from yellow light due to the ratio of R and G response.

It is the brain that interprets color. Beauty in the eye of the beholder.4 When the R receptor gets about the same signal as the G receptor, or R = G, yellow is perceived. When the R receptor gets a larger response the the G receptor, something like R = 3G, red-orange is perceived. It is the brain that interprets color.

THIS IS NOT YELLOW Magenta is not a real color. Biology Philosophy

Magenta is a non-spectral hue.

Maxwell’s Triangle This triangle was devised to illustrate how three primary colors —Red, Green,Blue— can be added together to generate the other colors. At the center of the triangle is white. The colors in the triangle can assigned three coordinates, like a vector, determined by how much red or blue or green is mixed to make that color. For example, a saturated red added to a saturated green makes …yellow. Red and green make YELLOW??? Yup. We’ll show you how.

Maxwell’s Triangle Think of the saturated red and green colors on Maxwell’s diagram as vectors. Now, imagine “decomposing” these R and G vectors into the sum of the dashed arrows.

Maxwell’s Triangle See how two of the decomposed vectors that run along the right edge of the triangle are co-linear but point in exactly opposite directions? They cancel each other. This leaves the shorter vectors components these vectors have the same direction and point from yellow. They represent the yellow “product” from adding red and green.

Maxwell’s Triangle Blue + Yellow = White?! Blue + yellow vectors point in exactly opposite direction and cancel to make white.

In the early 1800’s, Ogden Rood physicist and Michael-Eugene Chevreul chemist ebxplored optical mixing.

CIE Chromaticity curve Maxwell’s triangle excludes some colors that are visible to eye. The chromaticity diagram is meant to show extra colors visible to the eye. Pink at x=0.34, y= 0.23

A mid-tone flesh pink Unsaturated, non-spectral hues are especially likely to be found as metameric mixtures.

Color wheels for mixing paint to neutralize color: OK. yellow green red cyan magenta Not OK for assigning visual compliment in spectroscopic analysis of compounds !! blue

blue + yellow = green One light beam!!!! Subtractive color mixing: light colors are subtracted from one white light beam blue + yellow = green white light beam = sum of all spectral colors reflects this much spectrum light reflected By both blue paint shining on: reflects Green is only color reflected by both the blue and yellow paints. All red, orange, yellow, blue, violet removed. yellow paint One light beam!!!!

A transmittance —or reflectance— spectrum reflects this much spectrum ultramarine light reflected by both reflects chrome yellow

blue + yellow = green One light beam!!!! white light beam = sum of all spectral colors This blue reflects no violet. light reflected by both blue paint (cerulean) shining on: reflects a brighter green, less dull, because the (violet + orange = brown) is absent yellow paint (lemon yellow) This yellow reflects no orange. One light beam!!!!

reflects this much spectrum Cerulean blue reflects lemon yellow light reflected by both reflects lemon yellow

Light “hues” removed from one beam So the primary colors are not Red-Yellow-Blue??!! Primary Colors for Color Mixing Additive Multiple light beams added Subtractive Light “hues” removed from one beam primary colors Y-M-C yellow/magenta/cyan Y + M + C = Black primary colors R-G-B red-blue-green R + G + B = White secondary colors Y-M-C secondary colors R-B-G Demonstrate this: with 3 slide projectors Demonstrate this: with transparencies

(some) Color Terminology common idea scientific term Hue green Lmax, nm or Do what color is grass? Saturation how intense the color, e, absorption coefficient relative to grey Luminosity relative brightness concentration (Value) how much white/black is added (e.g., molarity) Non-spectral hues are not in the rainbow! are not a component of white light examples: are not due to one wavelength of light brown, salmon, magenta, purple, pink

or saturation

How does all this relate to spectroscopy of transition metals complexes?

Conversion between the two primary color systems, Red-Green-Blue and Cyan-Magenta-Yellow illustrates the mathematical concept of equivalent basis sets and their interconversion Examples of changing 3-dimensional basis sets: Color space: RGB CMY Geometrical Space: Cartesian Coordinates (x,y,z,) into Spherical (Polar) Coordinates (r, ) Chemical “Space”: Atomic orbitals and molecular orbitals

philosophy art math history COLOR biology chemistry language

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