Stupid Question ™
April 12, 2001
By John Ruch
Q: Why do trees look green up close, but blue when seen on a distant hillside? How far away do they have to be for this color change to take place?
—Mark E. Wendel
A: The majesty of purple mountains is apparently due to the scattering of light waves by atmospheric dust and moisture, and the air molecules themselves.
I say “apparently” because light is a fickle superstar, famous for beavhing as both a particle and a wave, and currently starring in the hit mystery Quantum Theory. Anything I say about light will probably be considered ridiculous in a hundred years; meanwhile, I will stick with the “wave” metaphor for convenience, and because it historically made the answer to this question possible.
Visible light is part of the electromagnetic spectrum, moving through space in the form of a wave. The colors we see are distinguished by various wavelengths of light, from the reddest red at c. 760 nanometers, to the most violet violet at c. 380 nanometers.
In the atmosphere, light waves start running into floating particles. If these particles are 1,000 nanometers or more across, they simply absorb the light. If they’re 1 nanometer or less, they have no effect on the light.
But anywhere between 1 and 1,000 nanometers, they scatter the light. These particles are stimulated by light’s electromagnetic field, which causes them to absorb the light, then re-emit it in a different direction. Shorter wavelengths—the blues and violets—are more easily scattered (violet 16 times more than red), just as in water small waves are more easily reflected than large waves.
Repeating billions of times per second, this scattering makes the shorter-wavelength colors predominate in the atmosphere. Indeed, scattering is why the sky looks blue. (Why not violet, the shortest wavelength? Because the upper atmosphere absorbs much shortest-wavelength light, and our eyes are less sensitive to it.)
The sky’s blueness was a mystery prior to the wave theory of light. Einstein demonstrated that air molecules are mostly responsible for the scattering in the sky.
With land objects, a bluish color usually has more to do with floating foreign particles. A forest that’s green up close will look bluish or purplish in the distance mostly because of dust and humidity that cause this scattering to occur and short-wavelength light to predominate. (Scattering is also why the trees will look blurry.)
The Blue Ridge Mountains of the Appalachians are so named because of this phenomenon, which there is caused by high humidity and gases given off by conifer trees.
Obviously, the blue-coloring effect of scattering is negligible over short distances (though other scattering effects are visible at close quarters, such as shadows being merely shady rather than pitch black).
Since it’s a cumulative effect dependent on atmospheric conditions, it’s hard to say exactly how far away you must be for something to appear bluish; a mile might suffice on a foggy day, while 10 miles might produce no effect in clear mountains.