Mixing Colors and Infrared Light

By Dooling, Dave | The Science Teacher, July 2006 | Go to article overview

Mixing Colors and Infrared Light


Dooling, Dave, The Science Teacher


Byline: Dave Dooling

Question:

Why do yellow and blue make green when we mix colors?

Diane Langmuir, Dwight-Englewood School, Science Teacher, Englewood, New Jersey

Answer:

To answer this, we have to distinguish between color addition, the mixture of colored lights into a shared beam, and color subtraction, the filtering of certain wavelengths of light by reflection off pigments or transit through gels. Sunlight provides a good example of color addition; radiation from all the visible regions of the spectrum mix to form white light. We can split white light into its constituent spectrum by passing it through a prism or reflecting it off a diffractive surface such as a CD. "Roy G. Biv," the most famous nonexistent scientist in modern physics, inspires us to remember the order of the colors of this rainbow-red, orange, yellow, green, blue, indigo, and violet.

We perceive color as our brain analyzes the frequency of the light (or light mix) landing on our retina. Three types of rhodopsin, a photoelectrically sensitive protein, report the intensity of three different bands of visible light to our brain by sending nerve signals through the optic nerve. Our perception of color is the mind's interpretation of the different wavelength ranges, which we perceive as red, green, and blue. As we view the world, different combinations of red, green, or blue cone cells trigger to produce the essentially infinite range of colors we perceive when light is reflected from objects and into our eyes.

The primary colors of light are red, green, and blue. If we were to aim red light onto a screen, and overlap it with a green light, the shared region would appear yellow, corresponding to an area of the spectrum around 555 nm in wavelength, a region between the peaks for red and green. Red and blue light combine to form magenta; and blue and green form cyan. Thus, the addition of two primary colors of light produces each of the secondary colors; and of all three, white.

A different process is at work with paints. The pigments in yellow paint reflect red and green light, while absorbing blue. Blue paint reflects blue light and some green, absorbing red. The resulting mixture absorbs all colors except the mutually reflected green. The four-color printing scheme uses cyan, yellow, and magenta, which can be combined to form all colors by color subtraction; and black, necessary in practice to create the darker hues. …

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