Chapter 27

Color

When light strikes an object, the wavelengths are affected in one of two ways:

  • If the wavelengths are near the natural frequencies of the material, those wavelengths are absorbed.
  • If the wavelengths are not near a natural frequency, those wavelengths are reradiated.

The combination of these two processes determines the color of the material. So a blue shirt takes on that color because wavelengths that are not blue are absorbed. This same explanation can be used to understand transparent colored material. For example, a piece of green transparent filter material must absorb all wavelengths but green. The greens are transmitted.

We can create a simple scheme to work through color mixing. We begin with the blackbody curve of the Sun, which for humans defines white. But let’s simplify the actual blackbody curve in two important ways:

  • Experimentally we can confirm that only three primary colors are needed to obtain all other colors. The three additive primary colors are red, green, and blue
  • Let’s assume that the radiation curve of the Sun gives equal amounts of the primary colors.

The purpose of these assumptions is so we can treat the radiation curve of the Sun as if it were three equal blocks, one of each of the primary colors.

We need to memorize the result of adding two of these

primary colors, i.e., what is the result of shining spotlights of

these colors onto the same white surface.

Red + Green = Yellow

Red + Blue = Magenta

Green + Blue = Cyan

The resultant colors are called the subtractive primary colors.

Since the presence of all three primary colors simulates sunlight, then R + G + B = White. We define complimentary colors as any two that will add to produce white. Using the primary colors, we see that

Red + Cyan = WhiteBlue + Yellow = WhiteGreen + Magenta = White

The model for color subtraction is as follows:

  • First indicate the color of the illuminating light by combinations of three arrows for the three primary colors. Thus indicates white light, but indicates a yellow light.
  • Indicate the color of the object (dye, paint, chalk, etc) in white light as a series of three colored blocks, one for each primary color. Show X’s in the block(s) where the light is absorbed. Thus would represent a paint that absorbs reds while reradiating blue and green. The paint will look cyan, which is the addition of blue and green.
  • Since the X’s indicate a color is absorbed, simply pass the arrows that exist through the blocks that do not have X’s. The addition of the result is the color.