Since we spend most of our lives immersed and surrounded by physical objects, most of what we see consists of objects illuminated by our Sun or some other radiative source, like a lightbulb or a fire.
Surprisingly, the eye has evolved in order to see radiative light, as opposed to reflective light, as if we evolved to look at a direct source of light, rather than the secondary information.
The cones in the back of our eyes are red, green, blue sensitive, which are receptors for direct emission information.
What is the difference and why is it important?
Radiative light comes from a direct light source, like the Sun or a fire, or a TV, or a computer screen.
Reflective light comes from a secondary source, like light bouncing off of a table top, a tree or a bowl of fruit.
The spectra of the light we have evolved to see is radiative light. It consists of emission lines, bright narrow lines that cross through the spectrum. Reflective light has absorption lines, dark lines that run through the spectrum.
These differences in light are significant when it comes to being processed in our brain. This can be observed when we sit in front of a fire, a TV or a computer, where we typically become physically inactive, in that we stare almost undistracted and hypnotized into the source of radiation.
Reflective light are photons bouncing off of surfaces, inheriting and changing the radiative source depending on the pigment of the surface.
For example, if you are looking at a red tablecloth, the tablecloth is really not red; it is every other colour other than red. The other colors have been absorbed by the material and the red has been ‘spat out'; it is this information that reaches our eyes.This secondary radiation is red yellow blue rather than red, green, blue.
From an evolutionary standpoint, our eyes would be much better suited to first and foremost interpreting secondary light. Imagine a lion chasing you across a savannah. The information you are dealing with to survive is primarily secondary light (yellow red blue,) light bouncing off of surfaces.
That the cones in our eyes would have evolved red green blue, which first and foremost deal with a direct light source, seems unintuitive.
But there may be an answer to this.
Eyes in their primordial form would have evolved in water, and probably were initially only able to distinguish light from dark. If you look up through the water, the primary source of light is the Sun, a direct radiative source.
So it might be that the eye evolved by determining a predator by absence of light caused by the its presence. This would explain how our eyes evolve the way they did; shadow.
Stephen Goodfellow 07/14/2016