in a recent blog post¹, a friend of mine asked me if white was a color... the answer is simply - yes... white is a color... this blog post extends to discuss the science of seeing color!

colorful, isnt she?

about light

to a physicist, light is electromagnetic radiation... why? 'cos its too long to say / type "electromagnetic radiation"... :) well no 'cos its got to do with this photon thing...

light is composed of an elementary particle called the photon... from the wave-particle duality theory from quantum mechanics - it has (relativistic) mass and hence momentum, energy and it also has oscillating electric and magnetic fields and therefore a "frequency"...

in plain english - light has a frequency and it has mass - just like a spinning cricket ball!

colorful light

light consists of photons... and a photon has a distinct wavelength / frequency... we can categorize light depending on the frequency / wavelength of the these photos... the human eye is sensitive to light corresponding to a range of wavelengths (400-700nm)‚ and we call this range of light, visible light... the diagram bellow² shows the spectrum of light as we know it...

oh and if our eyes could see more than just visible light, we'd be wearing space suits... if not we'll look naked! :p

a light ray consists of a single photon of a distinct frequency... that is... a red ray of light would have a photon of wavelength 400nm and a corresponding energy level given by planks equation: E=hf (where E is the energy, h is planck's constant, and f is frequency)... it does not have multiple frequencies...

white

there is no such thing called white light‚ you wont find a photon corresponding to 'white' light‚
but you can find a collection of photos of various frequencies which your eye would call 'white' light‚

max plank modeled the relationship between the energy density (energy per volume), frequency (color) and temperature of light emitted from a source called a black body‚ a black body does not absorb or reflect any light - it only emits light... and he gave a set of equations to analyze energy (heat) transfer from colorful light emitted by various sources - like the sun!

sunlight is the spectrum of light given off by the sun... the sun is quite far away from the earth and when modeled based on his equations we eathlings find the sun to be a black body of temperature 5500K (around 5250°C) ... so we say that sunlight corresponds to 5500K...

and a light source at around 2000K (around 1750°C) would appear to be orange / red...

our main source of light being the sun emits light corresponding to a black body at 5500K and we call this white light...

pigments

pigments are materials which absorb light but allowing some light of distinct wavelengths to be reflected... a red pigment would absorb sunlight which is a collection of light rays of various wavelengths but reflect the red light rays...

the amount of colorful light present in the light source would determine the color properties of the light reflected off the pigment

the eye

the human eye has cones and rods to sense visible light
cones see color while rods dont and there are three types of cones
each type is sensitive to a light of varying ranges‚ (red, blue and green - the primary colors)

In 1965 came experimental confirmation of a long expected result - there are three types of color-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors.³

when light falls on these cones... a chemical reaction takes place... triggering a nerve impulse... and then our brain processes the nerve single...

The physical process of seeing begins with the absorption of a photon by a pigment called "retinal". The absorption causes a change in the geometry of the retinal (in the terms of organic chemistry, it changes from a "cis" to an "all-trans" configuration). This change in geometry takes place in about 6 ps. It has the effect of activating an enzyme (phosphodiesterase) which in turn hydrolyzes hundreds of "cylic-GMP" molecules (guanosine 3' - 5' phosphate) which in turn effects the sodium channels in optic nerves, initiating a nerve impulse.

(Note that at the level of proteins, geometry defines function; this business of shape-changing, as well as "key and lock" shape fitting, is a ubiquitous signaling mechanism in the world of molecular biology.)

The specific protein to which the retinal is linked determines the range of wavelengths to which it is sensitive. Retinal linked to rhodopsin has a peak sensitivity of about 496 nm. Rhodopsin is the protein in human rod cells as well as in many photoactive organisms.⁴

color mixing

so now we know that our eyes see red, blue and green as primary colors...

so what happens when you mix a blue pigment with a yellow pigment - the blue will try to reflect all light other than blue and the yellow will try to reflect all but yellow... so in theory you should get black!

well not really... lets take the primary subtractive colors or the supposed primary colors as we learnt in school during art class: cyan, magenta and yellow

cyan in terms of primary colors is a mix of green and blue
magenta in terms of primary colors is a mix of red and blue
yellow in terms of primary colors is a mix of green and red

cyan pigments absorb all colors except green and blue

yellow pigments absorb all colors except green and red

so when we mix cyan and yellow pigments.... the mixed pigments absorb blue and red allowing green to be reflected....

primary colors

science would suggest that red, blue and green are the primary colors
because thats how our eye sees color

i know that in art class we learnt that cyan, magenta and yellow are the primary colors but the basis for it is not biological or even physical... the art and science of recreating natures colors is something we've been trying to do since we started eating cooked meat!

red, blue and yellow are primary 'mixing' colours when you’re dealing with paint (not light).

it‚Äôs important to teach children basic concepts that don‚Äôt contradict what they will learn later‚ but on the other hand, learning about rods & cones re: colour vision is a bit useless when all they want is to be able to paint an orange dog, and they only have five ‚Äòcolours‚Äô of paint (red, blue, yellow, black & white)

you can make green with those primary colours of paint (red, blue, yellow) but you can’t make yellow if your 'primary colours' are red, blue & green. so kids get taught that green is a secondary colour.

its not like kids have three colors to paint with... and its not like they would paint mixing these colors... i've never seen kids mix colors to paint... they mix colors for other reasons... like when they are bored or something...

and teaching kids false science is not cool!a its like saying some monster would come eat you up if you dont eat your vegetables! look what that did to me... i cant stop eating!

references

1. Curly Tales: Is white a colour?
2. Wikipedia Image: EM Spectrum
3. The Color-Sensitive Cones
4. Spectral Sensitivity of the Eye - Kenneth R. Koehler
5. Wikipedia Image: Solar Spectrum
6. Wikipedia Image: Color temperature