Color
The Relationship between Absorption and Appearance

The following simple illustration is meant to help explain why a material that appears one color actually is absorbing light of a different color. While a piece of green glass is used in this example, the logic obviously applies to any type or color of material.

Nature of Visible Light

The color of light is determined by its wavelength, typically expressed in either nanometers (1 nm = 10-9m) or Angstroms (1 A = 10-10m) for visible light. The visible range is approximately 400-750 nm. Larger numbers indicate longer wavelength, which is actually lower in energy. For example, violet light (~400 nm) is higher in energy than red light (~740 nm). Light can either be a single wavelength, or (more commonly) a combination or range of wavelengths. For example, the blueish light of a argon ion laser has a single wavelength of 514.5 nm. However, white light contains light of all wavelengths in the visible region (and beyond).

In the following illustration, for simplicity, we will assume that white light is composed of only three colors (red, blue, and green). This is how colors are often defined in computer and television displays. The following table shows some colors, very approximate wavelength of "pure" light having this color, and the RGB definitions used to create each color.

? ~740 nm ~560 nm ~480 nm
red = 0%
green = 0%
blue = 0%		 red = 100%
green = 0%
blue = 0%		 red = 0%
green = 100%
blue = 0%		 red = 0%
green = 0%
blue = 100%

 

~600 nm ~400 nm ~510 nm ?
red = 100%
green = 100%
blue = 0%		 red = 100%
green = 0%
blue = 100%		 red = 0%
green = 100%
blue = 100%		 red = 100%
green = 100%
blue = 100%

Absorption and Color

The colors discussed above only apply directly to things that give off (emit) light. Most colored objects around us do not actually give off light, but simply reflect light that is already present. (Turn off all other lights and you can't see most objects). The colors of these objects is due how light "bounces off" or "shines through" the object.

Consider a simple piece of green glass. If we shine our "white" light through this piece of glass, we see green. The following diagram shows why.

¾® Red light ¾®   Green
Glass		   (red light absorbed by glass)

Observer
(sees
green)
   
¾® Green light ¾® ¾® Green light ¾®
   
¾® Blue light ¾®

(blue light absorbed by glass)

If the glass were colorless, no light would be absorbed and the "color" observed would simply be the color of the light shining on the sample (white). In the case of the green glass, the red and blue components of the light are absorbed. However, the green light passes through unchanged. The observer sees only the green light that is "untouched" by the glass.

While this discussion focussed on transmittion of light through a sample, the same principles apply to light bouncing off of the surface of an opaque object. One example of an application of this is the relationship between the color of a car (and its interior) and the temperature the interior gets on a hot, summer day. The light that is absorbed is turned into heat, so the darker the color, the more light absorbed, and the hotter the car gets.