All About Prisms, Light, and Color!

Color is all around us. Everywhere we look, our brains process millions of colors almost automatically. But what actually is color?

To answer this question, we first have to investigate a broader question: what is white light? In the late 17th century, Isaac Newton sought to uncover the relationship between light and color.

Around this time, many thought that color originated from light mixing and mingling with darkness. Following this theory, it was proposed that light mixed with the least amount of darkness would produce red, and darkness with a splash of light would produce blue.

Newton disagreed with this theory. To disprove it, he created a hole in one of his shutters. Then, placing a glass prism, much like the one featured in our video at the top of this lesson, in front of the hole, Newton projected a rainbow onto a white piece of paper. He then was able to refract the colors of the rainbow back into white light, proving that the prism was not coloring the light.

White light being refracted through a prism

Therefore, Newton proved that within white light, all of the colors are present. So how is it that when we look at certain objects, like oranges, the color orange is what we see?

This is because of something called photons. Light is, in basic terms, the movement of photons. The more photons there are, the more intense the light. Photons move in wavelengths, which vary based on the amount of energy the photons possess. The higher the energy of the photon, the shorter its wavelength and the higher its frequency. This produces blue and violet. Similarly, the lower the energy of the photon, the longer its wavelength and the lower its frequency. This produces red and orange.

So now that we know that photons travel in varying wavelengths, how does this help us figure out our orange conundrum?

Well, when we look at an orange, all other wavelengths of light are absorbed except orange, which is reflected back to our eyes (see image below).

Orange reflecting orange and absorbing other colors

Therefore, our eyes detect the orange wavelengths, but not the others.

How does this detection process work, you may ask? Well, light first enters our eyes through the cornea. The cornea bends light through refraction, much like a prism, toward the pupil, which monitors how much light makes contact with the lens. The lens then focuses the light on the retina.

Side view of an eye

The retina is made up of rods and cones. Rods are activated in low light situations, and cones are used when there is bright light.

A closer look at rod and cone cells

Cones have photo pigments that detect color. Humans typically have three that detect red, green, and blue wavelengths. For example, if you were looking at an orange, the cones that would be activated to let you know that the orange is orange are the green cones, slightly, and red cones.

The cones transmit an electric signal along the optic nerve to the visual cortex in the brain. This area of the brain will then process the number of cones that were activated and the strength of their signals to allow you to see the orange as orange.

Visual cortex in relation to rest of brain

Sources:

https://www.aao.org/eye-health/tips-prevention/how-humans-see-in-color

https://www.asu.edu/courses/phs208/patternsbb/PiN/rdg/color/color.shtml

https://www.college-optometrists.org/the-college/museum/online-exhibitions/virtual-observatory-gallery/newton-and-the-colour-of-light.html

http://www.webexhibits.org/colorart/bh.html

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