How do humans see light and color?

The human visual processing system provides critical information that has enabled the human species to survive and interact with the world around us. For example, perceiving a patch of green plants in a desert landscape tells us where water can be found; distinguishing different shades of red tells us when fruit is ripe enough to eat.

To see our environment, the human eye contains light receptors called rods and cones, as shown in the figure below. We see because light strikes the rods and cones of the retina, which fires the proteins rhodopsin and photopsin. This instantaneous chemical change sends signals via the optic nerve to the brain. The brain then processes those signals as images.

Rods and cones serve different functions: rods enable us to see in low-light conditions, referred to as scotopic vision, but they provide limited or no color perception. Thus, dusk and nighttime scenes appear to us with limited color differentiation in shades of gray and black.

The cones are responsible for our daytime light sensing—called photopic vision—and for our color perception. There are three types of cones: S, M, and L, referring to their specialization to receive short, medium, or long wavelengths of light. They are also often called blue, green, and red, the colors that correspond to their respective wavelength spectra, although each type of cone can perceive more than one color.

The Visible Spectrum

Light visible to humans is just a sliver of the full spectrum of electromagnetic radiation. This radiation is typically described by its energy or, inversely, by its wavelength (measured as the length from the peak of one wave to the peak of the next in meters/millimeters/nanometers).

The entire electromagnetic spectrum extends from short wavelength, high energy gamma rays on one end all the way to long wavelength, low energy microwaves and radio waves on the other. A small band in the middle of the spectrum is the range of light visible to the human eye. Visible light has wavelengths within the range of approximately 380 nanometers (nm) to 750 nm, bounded by ultraviolet (UV) at the low end of the range and infrared (IR) at the high end, as shown in the image below.

Sunlight is an example of full spectrum light: it contains all the wavelengths (all the colors of light) blended together. We see this as white light. Humans perceive color of objects and the environment because those objects absorb certain wavelengths of light while reflecting others. The reflected wavelength is received by our eyes and interpreted as a color. For example, a red apple absorbs most of the violet, blue and green wavelengths, reflecting primarily reddish wavelengths, thus we perceive the apple as red.

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