The Color of Light

Without a human eye and brain there is no color

A workshop given in San Jose, Costa Rica, for Cientec, by Paul Doherty

Human perception of color is complex. Three different kinds of cells in the human retina, called cone cells, contain three different molecules that resonate with different frequency ranges of light. These molecules convert light energy into electrical nerve impulses that are sent through the optic nerve to the visual cortex at the rear of the brain. There the electrical impulses are converted into a perception of color. Thomas Young guessed that there were three color detectors in humans circa 1800, scientists actually isolated these chemicals from human retinas in the mid 1900's.

Different animals have different chemicals to detect light. Some birds that pollinate flowers have less ability to detect blue light, while bees have great ultraviolet detection but poor red detection. In this first activity you an gain an appreciation of how the world might appear to color blind people, or to other animals.

Red vs. Green, look at color photos through red and green filters.

Red-Green afterimage, look at a light through red and green filters then look at the world.

Scientists analyze light by separating it into a spectrum. They look at the amount of energy at each different wavelength of light. Light that is seen as red by a human has a wavelength that is just about double the wavelength of light that is seen as blue. Isaac Newton separated sunlight into its spectrum using a prism in 1670. You can do the same thing with light from an overhead projector and a compact disk today.

CD Spectra, look at lights reflected in a compact disk.

CD spectrometer, make a spectrometer with a compact disk.

Spectra of common light sources.

For teaching, it is useful to be able to project a spectrum onto a white wall inside a darkened classroom.

Project a spectrum, use a compact disk and an overhead projector to project a spectrum.


Most colors in the world come from pigments, chemicals that turn some wavelength ranges of light into heat and allow other wavelength ranges to be transmitted or reflected. The spectrometer is a "truth teller for light." You can see what pigments in colored plastic, food coloring, or even paints and dyes do to light by looking at these colors with a spectrometer.

Explore spectra using the overhead projector

Spectra exploration

Color without pigment.

Clear materials when they form thin layers, that is layers that are as thin as a wavelength of light reflect some colors well and other colors not a all. These are called interference colors. They are colors that are produced by pigment free clear materials.

Soap Film in a can, look at the colors reflected by a soap film across a film can.

You can also create permanent interference colors by creating a thin layer of clear nailpolish on a black piece of paper.

Interference Colors from clear nail polish. Thin layers of clear materials can reflect colors by interference of light waves reflected from the front and back of the layer.

So, in this lecture we have looked at the human eye and its perception of color using chemicals on the retina to convert light into electrical impulses in neurons. Then we have taken light apart using a spectrometer made from, a compact disk, a "Truth Teller for light." Then we used our spectrometer to look at colors produced by pigments, chemicals which turn some frequencies of light into heat. And finally we looked at how colors can be reflected by clear materials through interference of light waves. we have met many new and interesting ideas, now it is time to go out and do the experiments which accompany this lecture. Have fun exploring science.

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Scientific Explorations with Paul Doherty

© 2003

21 August 2003