In determining color, our brains tend to ignore the color of the light source. But what if there is more than one light source, each with a different color temperature?
Blue sky illuminates rock; sunlight illuminates reflected trees. Photo: © Donald J. Rommes
Is the blue color of the rock natural or artificial? The Elk River widens and slows as it departs the mountains and approaches the ocean. Here, the flat surface of the shallow water is rippled by a transient breeze while reflecting the sunlit trees on adjacent hillsides. Meanwhile, a river rock in shade is illuminated, not by sunlight, but by an open blue sky. We don’t usually notice how blue the light is in the shade because our perception of color is automatically and subconsciously determined by comparing the relative colors of neighboring objects, not by the color of the light illuminating them. If we visited this place on a blue sky day after sunset, everything would have the same blue color cast, the relative color differences would be maintained, and we would perceive the rock as gray.
However, the primary light source in this photograph is the daylight from the reflected trees. The river rock is the exception because it is illuminated only by the blue sky. Relative to the reflected trees, it is much bluer and is perceived as such by our brains.
Seeing a blue rock in sunlight is unusual: it creates interest and perhaps skepticism, but it is a natural phenomenon with an explanation rooted in human physiology.
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