Blue Sky, Green Leaves

October 23, 2005

Sandy_2005_blueandgreen copy

Provided and copyright by: Rob Sheridan
Summary authors & editors: Rob Sheridan

On a sunny day, our retinas receive blue wavelengths from the sky and green wavelengths from the foliage, but for different reasons. The blue color of the sky is due to Rayleigh scattering. Long wavelengths (red side of the visible electromagnetic spectrum) pass through the atmosphere relatively undisturbed. However, the shorter wavelengths (blue side of the visible spectrum) are more readily scattered by the gas molecules comprising our atmospheric. Actually, there's a continuous range of scattering cross sections that increase from the red end of the spectrum to the ultraviolet. Keep in mind that although a single photo is scattered in only a single direction, multiple photons are scattered in multiple directions. On a sunny day, in whichever direction you look (except directly at the Sun or close to the horizon), some of the blue light scattered by the atmosphere reaches your retinas. Hence, the sky appears blue.

Foliage appears green because chlorophyll, the molecule that catalyzes the reaction in which carbon dioxide and water combine to form glucose and oxygen, absorbs long wavelengths (red side of the spectrum) and allows middle and short wavelengths (greens and blues) to pass through or be reflected (toward our retinas). The glucose created can be oxidized (for immediate energy) or polymerized to starches (to store energy). Chlorophyll is a photoreceptor molecule, very similar in structure to hemoglobin. It's essentially a porphyrin ring surrounding a single molecule of magnesium (hemoglobin replaces this with a single molecule of iron). In autumn, the chlorophyll in leaves of deciduous trees decays, allowing the orange and red carotenoid pigments to show.

In essence, the physics of atmospheric scattering make the sky appear blue, whereas, the chemistry of chlorophyll’s absorption spectrum makes the foliage appear green! The photo above was taken in central New Hampshire on September 5, 2005.

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