The experience of seeing color, such as red, is a complex biological and physical process. It begins with the light source and the physical properties of an object, culminating in a sophisticated interpretation by the human brain. The journey from a light wave hitting an object to the subjective perception of “red” involves specialized cellular hardware and intricate neural coding.
The Physics of Color and Wavelength
Light is a form of electromagnetic radiation, and the portion visible to the human eye occupies only a small band of the entire spectrum. The color we perceive depends directly on the wave’s length. Longer wavelengths correspond to colors at the red end of the spectrum, while shorter wavelengths correspond to colors like blue and violet.
The color red is associated with the longest wavelengths in the visible spectrum, typically ranging from about 620 to 750 nanometers (nm). When white light strikes an opaque object, the object’s molecular structure absorbs most wavelengths. The color we see is the particular wavelength the object reflects back toward our eyes; a red object absorbs most green and blue light while reflecting the longer red wavelengths.
The Eye’s Light-Sensing Structure
The reflected light must first pass through the lens and strike the retina, a layer of tissue at the back of the eye containing light-sensitive cells called photoreceptors. There are two types of photoreceptors: rods and cones, specialized for different visual tasks. Rods are highly sensitive to low light levels but cannot distinguish between wavelengths, which is why we see only shades of gray in the dark.
Color vision relies entirely on the cones, which require brighter light to function and are concentrated in the fovea, the central part of the retina. Humans possess three distinct types of cones, named for the wavelengths of light to which they are most sensitive: Short-wavelength (S-cones), Medium-wavelength (M-cones), and Long-wavelength (L-cones). Each cone type contains a different photopigment, which determines its spectral sensitivity. These three cone populations form the biological foundation for perceiving the full spectrum of colors.
The Specific Mechanism for Seeing Red
The perception of red is primarily driven by the L-cones, which contain a photopigment that peaks in sensitivity toward the long-wavelength end of the spectrum, around 560 nm. Although L-cones are often called “red” cones, their peak sensitivity is actually closer to yellow-green. They overlap significantly in their response curves with the M-cones, which peak around 530 nm.
True red perception is not solely the result of L-cones firing, but rather a differential comparison of signals between the two long-wavelength cone types. When light is pure red, it causes maximum excitation of the L-cones while only moderately activating the M-cones. The visual system interprets this specific ratio—a high signal from L-cones relative to M-cones—as the color red, distinguishing it from yellow or orange, which cause a more equal activation of both L and M cones.
How the Brain Interprets Color
The signals generated by the three types of cones travel through the optic nerve to the brain, where they are processed according to the opponent process theory of color vision. This theory states that cone signals are organized into three opposing channels: red-versus-green, blue-versus-yellow, and black-versus-white for brightness. The specific relative activation of the L and M cones feeds into the red-green opponent channel.
When the L-cones are strongly stimulated and the M-cones are less so, the signal is sent as “red,” while the opposite ratio signals “green.” This mechanism explains why we can see a yellowish-red or a bluish-green, but never a reddish-green, because the two colors are processed antagonistically by the same neural pathway. The final subjective experience of red is constructed from these opponent signals within the lateral geniculate nucleus and the visual cortex of the brain.