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Why do different cone cells absorb different wavelengths of light?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

Different cone cells absorb different wavelengths of light due to variations in the amino acid composition of their opsin proteins.1

Each cone cell contains a photopigment consisting of an opsin protein bound to a chromophore molecule. While the chromophore is generally the same across these cells, the specific amino acid sequence of the opsin alters the charge environment surrounding the chromophore. This change in the local chemical environment shifts the wavelength of light to which the photopigment is maximally sensitive.1

In humans, these variations result in three distinct types of cones—often designated as short (S), medium (M), and long (L) wavelength cones—which are maximally sensitive to blue, green, and yellow-green (red) spectra, respectively. By responding to these different wavelengths with varying effectiveness, the cones allow the brain to interpret a wide range of colors based on the ratio of excitation across the different cell types.12