By Mark Spencer, PhD, President, Aquametrix
When we humans look at objects we describe them in terms of color. We say that an apple is red or a leaf is green. To us color is an attribute just as surely as the mass or dimensions of an object. But the fact is that color is a pure human construct. Take away the human and it does not exist.
What we perceive as color is really just our brains interpretation of the range of wavelengths that the object reflects or (like Christmas tree lights) emits and that our retinas perceive. When a beam of white light strikes a tree leaf, the chlorophyll absorbs most of the red and blue wavelengths. The wavelengths of light that lie in the middle of the visible spectrum—i.e. the green wavelengths—are not absorbed and, hence, reflect off the leaf and into our eyes. The absorption of light at different wavelengths is complicated and different for every object. An absorption spectrum is a plot of the absorption of light over a range of wavelengths. The figure below shows the absorption spectrum of chlorophyll. Because every chemical species has a unique absorption spectrum its spectrum serves as a fingerprint of that species. I might pick out a color swatch from a paint store whose color matches the chlorophyll in the leaf but I can guarantee that the swatch’s absorption spectrum is different. The absorption spectrum is an intrinsic property of chlorophyll.