## Color Mixing

Newton performed a simple but original experiment. Positioning a lens in front of the prism, he recombined the rainbow of colors, focusing them back onto a single spot. To his surprise, this mix of colors produced a white light "not at all sensibly differing from the direct light of the sun." Intrigued, Newton continued to experiment. He tried recombining only parts of the colored light by moving a slotted board in front of the prism. He found that if he allowed a band of yellow to project on the wall, and next to it a band of red, he saw a third color—orange—where they mixed. This third color was always an intermediate color that appeared between the other two on the rainbow.

What experiments did Newton perform with a prism? Where did the concept of "primary colors" come from?

Newton had discovered additive color mixing. Newton found he could create all different colors by adding together wavelengths from three primary colors : red, green, and blue.

"But wait!" you might say. "I was taught in art class that red, yellow, and blue are primary colors because they can be mixed to produce any other color, but no other colors can be mixed to produce them. So how did green get into Newton's scheme?" The solution is to realize there are two different types of color mixing. Adding different wavelengths together like Newton did with his prism experiment is called additive color mixing.

What is the difference between additive and subtractive color mixing?

The other way to produce different colors by mixing is called subtractive color mixing. In subtractive color mixing, wavelengths are removed from a mix. When you mix yellow and blue paint to get green, you are actually carrying out a form of subtractive color mixing. The pigments of the paint absorb yellow and blue wavelengths, leaving the middle range strongest, producing green. When you are mixing paints the primary colors are red, yellow, and blue. They are called primary colors because these colors can be used to produce all the other colors, and no other colors of paints can be combined to produce red, yellow, or blue. But this is only true when subtractive color mixing is used.

Television, on the other hand, uses additive color mixing. Red, green, and blue dots produce all other colors on a TV screen. Green is a primary color when you are talking about color video monitors (additive color mixing) but yellow is a primary color when you are talking about mixing paints (subtractive color mixing).

Write to Dr. Dewey at psywww@gmail.com.

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