Color is not paint smeared onto the outside world. There is no wavelength called 'red' anywhere out there. Color is an experience, built jointly by light as a physical phenomenon and by the eye, nerves, and brain that receive and interpret it. This article follows that first stage step by step: how light turns from a physical quantity into the experience of color.
Light is a wave in which electric and magnetic fields push each other across space — an electromagnetic wave. Its wavelength (or frequency) sets its character: longer wavelength means lower energy, shorter means higher.
The spectrum is wider than intuition allows. It begins with radio waves kilometres long, passes through microwaves and infrared, crosses the narrow band we see, and continues into ultraviolet, X-rays, and gamma rays smaller than an atom. In that enormous spread, the slice the human eye responds to is roughly 380nm to 700nm — barely a handful of its width.
Why this particular narrow band? Not by chance, but where two physical facts overlap. First, the sunlight pouring onto Earth is close to 5800K blackbody radiation, and its intensity happens to peak in the visible range. Second, Earth's atmosphere and water block ultraviolet (via ozone) and absorb much of the infrared, yet open an almost transparent 'window' across the visible band.
So this band is simply the light that reaches the surface most abundantly and most clearly. Life evolved its sensors to match the richest available signal. In other words, visible light is 'visible' not because the light is special, but because of how our eyes were built to fit it. If visible light were sound to a bat, to us it is color.
Light passes through the cornea and lens and forms an image on the retina, the inner wall of the eye. The retina holds two kinds of photoreceptor that convert light into electrical signals.
Rods number about 120 million and are extraordinarily sensitive, handling dim light and contrast but not color — which is why the world looks grey at night. Cones number about 6 million, work in bright light, and create color. Cones are packed densely in the fovea at the center of vision, which is why things look sharpest and most colorful when you look straight at them.
The starting point of color is, remarkably, just three kinds of cone. Each is most sensitive to long (L), medium (M), or short (S) wavelengths, with peaks near L 564nm, M 534nm, and S 420nm.
The key is that each cone does not sense a single wavelength but responds across a broad range, and the three curves overlap heavily. So a single cone can never tell you the color. When light arrives, how much each of L, M, and S responded — the ratio of those three numbers — is the essence of color.
The millions of colors we see all compress into this three-dimensional signal. That color is fundamentally 'three numbers' is also the root of why the CIE color system in Part 2 begins from three tristimulus values.
So how does a cone turn a single grain of light — a photon — into an electrical signal? The protagonists are a protein called opsin and the retinal molecule nested inside it.
Normally retinal sits in a bent shape called 11-cis; the instant it absorbs a photon it snaps to a straightened all-trans shape (isomerization). That tiny shape change activates opsin and triggers a chain: transducin → the enzyme PDE → a sharp fall in cGMP. As cGMP drops, ion channels (CNG channels) in the membrane close and the cell hyperpolarizes.
What is striking is that light does not switch the signal 'on' but 'off'. A current that flowed in darkness is reduced by light, and that very 'change' becomes the signal. The event of a single photon is greatly amplified through a molecular cascade and born as one neural signal.
Cone and rod signals are first processed within the retina through bipolar and ganglion cells. Here the L/M/S values are not passed on as-is but recoded as 'differences' — red–green (L−M), blue–yellow (S vs L+M), and light–dark (L+M). This opponent structure is why we can never imagine a color that is 'reddish and bluish at the same time'.
The ganglion axons bundle into the optic nerve; at the optic chiasm the left and right information is rearranged, then relayed through the LGN to the primary visual cortex (V1) at the back of the head. From there the brain integrates form, motion, and color to finally create the experience of 'seeing'.
In the end, red was never out there in the world. It is a conclusion the brain synthesizes from a physical quantity as it passes through eye, nerve, and brain — the result at the very end of all this.
That is the mechanism by which light becomes the experience of color. But to ask whether 'the red I see is the same as the red you see', color must be defined not in words but in numbers. In 1931, humanity built the first coordinate system to quantify color. Part 2 unpacks, at the same depth, what experiment the CIE 1931 was born from, why negative values appear in the color-matching functions (the most confusing point), and how it evolved toward the more uniform CIE 1976, CIELAB, and CIEDE2000. → Read Part 2: Turning Color into Numbers