We think of color as an innate part of the world. An apple is red, a leaf is green, the sky is blue—all the evidence of our eyes suggests that we are seeing something fundamental about physical reality. And yet there are moments when color seems more like an illusion. If you sit in a space with green light for a while—say, a green plastic Porta-Potty—the world is suffused with red when you emerge. People looking at the same photograph of the same dress have radically different perceptions of its colors. At dawn and dusk, everything looks bluer, such that flowers that are red in sunlight are almost purple.
Your brain is up to something. But what?
In the case of the blue dawn effect, a pair of researchers, one at CalTech, the other now a post-doc at Harvard, recently published some experiments that may finally reveal what is happening.
Let's back up and start with some basic biology: When we say we “see” color, we mean that the light striking our retinas causes nerve cells to fire. Our brains compare the signals from three different types of cells to tell what to perceive. These cells each have a protein, called a cone, that will respond to short, medium, or long wavelengths of light; you can think of them as being a trio of musical instruments. The medium cone hoots loudest when medium-wavelength light strikes it. The others respond more quietly. From your perspective, each chord—each combination of responses—is a color.