Sunlight is the primary illuminator. Its spectrum is a familiar rainbow with maximum emittance in a color that most of us would call yellow-green — light of wavelength around 550 nanometers.

Different substances absorb, reflect and transmit different wavelengths of light that strikes their surface. The light that remains after that interaction determines the true color of the object.

What we see depends on the color spectrum of the illuminating light, the wavelengths of light that are not absorbed by an object, and a finely tuned perceptional mechanism in our visual cortex.

A red rose looks black in green light, and a green leaf looks black in red light. Other, more subtle differences appear when the incoming light spectrum is shifted only slightly from the sun’s.

Such is the case near sunrise and sunset as Earth’s atmosphere alters the light that reaches us. The atmosphere filters out much of the ultraviolet and scatters the blue, which gives the sun a yellowish appearance. As the sun gets lower in the sky, it passes through more atmosphere. This leaves more of the long wavelengths and shifts the spectrum further toward the red end and basking the landscape in that golden glow that photographers love.

Human perception of color depends on light sensors called cones on the retina at the back of the eye. Cones come in three flavors: red, green, and blue. Each sends electrical signals to the brain when stimulated by light in a particular range of wavelengths.

The human eye is not merely a camera. It works with the brain in a more complex way than the best digital camera to keep a constant “white balance.”

Color constancy is accomplished because perceived color derives from a comparison of the wavelengths reflected from the object and its surroundings.

In the warm light of dawn, a yellow lemon reflects more long-wave light and appears orange on film. But everything in its environment such as leaves also reflects more long-wave light. Like an extremely sophisticated digital camera, the brain compares the two and compensates for the color shift.

The subjective nature of color perception came under scientific investigation in the early 19th century.

Count Rumford (the American-born British physicist Benjamin Thompson) discovered to his dismay that when two different-colored lights illuminate an object, the shadow of the object takes the color of the other light.

He looked at the shadow through a tube that was painted black inside to eliminate glare from the surrounding lights and found that the shadows looked gray.

An assistant who was with him assured him that the shadows retained their color while both were observing them.

Later, John Dalton, the famous chemist and father of the atomic theory, discovered that blue flowers became yellow when he viewed them in artificial light.

A schoolmaster, Dalton tested the children in his classes and discovered that several of the boys saw the flowers the same way as he, while the remainder of the boys and all of the girls saw the flowers as red both outdoors and indoors.

Today we know that a certain percentage of colorblind, fair-haired boys lack the cones on the retina that are sensitive to red light. The defect, carried recessively on the Y chromosome, medically is called Dalton’s syndrome.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.