Colors are the smiles of NatureLeigh 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.

It’s not simple.It’s not real.It’s not universal.

Color wheels for mixing paint.

575 nm

400 nm

450 nm

520 nm

700 nm

But why should a color wheel work? The spectrum is linear!

Newton’s “color wheel”

Color wheels for mixing paint to neutralize color: OK.

Not OK for assigning visual complement in spectroscopic analysis of compounds !!

red cyan

green

magenta

yellow

blue

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:

If the electromagnetic spectrum were a piano keyboard…..

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

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

the visible spectral region would be just one key!

500 600 700 750650550450400

Each of the “colors” of visible light has a corresponding wavelength between 400 to 700 nanometers (or nm).

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.

565 nm

Light having a wavelength of near 565 nm will look yellow to most people.

Your eyes have only three color receptors (detectors)- the RGB cones (red-green-blue)

Beauty in the eye of the beholder.1

It is the brain that interprets visible light as “having” color.

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.

Beauty in the eye of the beholder.2

“What about red-orange?”, you say. “Red-orange light would also simultaneously stimulate the Red and Green receptors. See?”

Beauty in the eye of the beholder.3

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.

When the R receptor gets about the same signal as the G receptor, or R = G,yellow is perceived.

Beauty in the eye of the beholder.4

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 isa non-spectralhue.

Maxwell’s Triangle

This triangle was devised to illustrate howthree 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 colorson Maxwell’s diagram as vectors.

Now, imagine “decomposing” these R and Gvectors into the sum of the dashed arrows.

Maxwell’s Triangle

See how two of the decomposed vectorsthat run along the right edge of the triangleare co-linear but point in exactly opposite directions?They cancel each other.This leaves the shorter vectors componentsthese 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 physicistand Michael-Eugene Chevreul chemistebxplored optical mixing.

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

CIE Chromaticity curve

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.

Not OK for assigning visual compliment in spectroscopic analysis of compounds !!

red cyan

green

magenta

yellow

blue

blue + yellow = green

Subtractive color mixing: light colors are subtracted from one white light beam

white light beam =sum of all spectral colors

shining on: blue paint

yellow paint

reflectsthis much spectrum

reflects

lightreflectedBy both

Greenis only color

reflected by both the blue and

yellow paints.All red, orange,

yellow, blue, violet removed.One light beam!!!!

ultramarine

chrome yellow

reflectsthis much spectrum

reflects

light reflected by both

A transmittance—or reflectance— spectrum

blue + yellow = green

white light beam =sum of all spectral colors

shining on: blue paint(cerulean)

yellow paint(lemon yellow)

This blue reflects no violet.

reflects

lightreflectedby both

a brighter green, less dull, because the (violet + orange = brown) is absentOne light beam!!!!

This yellow reflects no orange.

Cerulean blue

lemon yellow

reflectsthis much spectrum

reflects

light reflected by both

So the primary colors are not Red-Yellow-Blue??!!

Primary Colors for Color Mixing

AdditiveMultiple light beams added

SubtractiveLight “hues” removed from one beam

primary colorsR-G-Bred-blue-greenR + G + B = White

primary colorsY-M-Cyellow/magenta/cyanY + M + C = Black

secondary colorsY-M-C

secondary colorsR-B-G

Demonstrate this:with 3 slide projectors

Demonstrate this:with transparencies

(some) Color Terminology

Hue green max, nm or o

what color is grass?

Saturation how intense the color, , absorption coefficientrelative 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 lightbrown, salmon, magenta, purple, pink

term common idea scientific term

or saturation

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

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

Conversion between the two primary color systems, Red-Green-Blue and Cyan-Magenta-Yellowillustrates the mathematical concept of equivalent basis sets and their interconversion

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The Apple Pickers - details - Pisarro

The Apple Pickers - details - Pisarro