The human eye perceives different colors based on the wavelength of light.
When light enters the eye, it is absorbed by a light-sensitive molecule called 11-cis-retinal, which is part of the rhodopsin protein in the retina. This absorption causes the 11-cis-retinal to isomerize to all-trans-retinal, leading to a series of events that ultimately result in the transmission of visual signals to the brain. The process of isomerization is initiated by the absorption of a photon of light in the visible range, which triggers the change in the shape of the retinal molecule from a bent structure to a more linear one. This structural change is essential for the activation of the visual signaling pathway. The interaction of photons with 11-cis-retinal and the subsequent cis-trans isomerization are fundamental steps in the process of vision[1][2][4][5].
Human vision is sensitive to a range of wavelengths from around 400 to 700 nanometers, with peak sensitivity at 555 nanometers in the green region of the visible light spectrum. The eye responds to a lower range of wavelengths between 380 and 650 nanometers, with the peak occurring at 507 nanometers. Humans exhibit trichromatic color vision, with significant response to red, green, and blue light stimuli. When all three types of cone cells are stimulated equally, it is the human brain that compensates for variations of light wavelengths and light sources in its perception of color. The longest wavelengths produce the perception of red, while the shortest ones produce the perception of violet. Different spectrally pure colors are said to have a different hue, and a spectrally pure or monochromatic color can be produced by a single wavelength[6][7][8]. Spectral purity is a physical dimension of light, where fewer wavelengths (colors) result in greater purity. This corresponds to the psychological dimension of saturation (amount of color).
The relationship between wavelength and color can be understood through the visible spectrum, where light with longer wavelengths is perceived as red, and light with shorter wavelengths is perceived as violet. The visible spectrum ranges from violet through red, with violet having the shortest wavelength. The human eye detects differences in wavelength as differences in color, with the longest wavelengths producing the perception of red, and the shortest ones producing the perception of violet[10].
The chemical difference between the three types of cone cells lies in the opsins they carry, which affect the absorption of retinaldehyde and result in different peak wavelengths of light sensitivity[11]. The three types of cone cells, which correspond to blue, green, and red photopigments, respectively are:
1. Short-wavelength sensitive cones (S-cones): These cones have a peak wavelength sensitivity in the range of 420–440 nm, the blue region of the visible spectrum. Expresses opsin: OPN1SW.
2. Middle-wavelength sensitive cones (M-cones): These cones have a peak wavelength sensitivity in the range of 534–545 nm, the green and yellow regions of the visible spectrum. M-cones make up about a third of the cones in the human retina. Expresses opsin: OPN1MW
3. Long-wavelength sensitive cones (L-cones): These cones have a peak wavelength sensitivity in the range of 564–580 nm, in the red region of the visible spectrum. The majority of the human cones are of the long type. Expresses opsin: OPN1LW
Genes make proteins. For example, the OPN1SW gene, also known as the short-wave-sensitive opsin 1 gene, makes the opsin OPN1SW.
Opsins are proteins that bind to light-reactive chemicals to underlie vision, phototaxis, circadian rhythms, and other light-mediated responses of organisms. They are found in various parts of the body, including the eyes, brain, and skin. In the context of vision, opsins are responsible for the conversion of a photon of light into an electrical signal in photoreceptor cells.
The different types of cone cells expressing different opsin proteins are responsible for detecting different colors in the environment. The presence of multiple types of cone visual pigments enables color vision[5]. The chemical substance responsible for human vision is retinal, which is present in all rod and cone cells[3]. Retinal exists in isomeric forms and is responsible for the absorption of light in the retina[3]. Isomers are molecules or polyatomic ions with identical molecular formulas, meaning they have the same number of atoms of each element, but distinct arrangements of atoms in space. So retinal exists in different shapes.
The sensitivity of each cone type to light depends on the specific photopigment they contain. For example, the green and red cones are mostly packed into the fovea centralis, with about 64% of the cones being red-sensitive, about 32% green-sensitive, and about 2% blue-sensitive.
In order to perceive color, at least two types of cones must be triggered, and the perceived color is based on the relative level of excitation of the different cones. The brain is responsible for interpreting the signals sent by the cones, and the combination of impulses from different cones determines the color humans perceive.
Summary
We see color through a complex interaction between the eye and the brain. The process begins with light reflecting off an object and entering the eye through the cornea. The iris adjusts the size of the pupil to control the amount of light that enters the eye. The lens focuses the light onto the retina, which contains specialized cells called cones. Cones are sensitive to different wavelengths of light and allow us to perceive colors. The cones transmit this information to the optic nerve, which sends signals to the brain’s visual cortex. The visual cortex processes these signals, allowing us to interpret and perceive the various colors in our environment.
Citations:
[1] https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Photoreceptors/Chemistry_of_Vision/Cis-Trans_Isomerization_of_Retinal
[2] https://www.pnas.org/doi/full/10.1073/pnas.2008211117
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2891588/
[4] https://www.sciencedirect.com/topics/medicine-and-dentistry/bathorhodopsin
[5] https://www.sciencedirect.com/topics/chemistry/11-cis-retinal
[6] https://www.olympus-lifescience.com/en/microscope-resource/primer/lightandcolor/humanvisionintro/
[7] https://en.wikipedia.org/wiki/Color_vision
[8] https://spie.org/publications/pm105_11_color?SSO=1
[9] https://academo.org/demos/wavelength-to-colour-relationship/
[10] https://www.britannica.com/science/color/The-visible-spectrum
[11] https://en.wikipedia.org/wiki/Cone_cell
[12] https://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html
[13] https://homework.study.com/explanation/color-vision-results-from-the-absorption-of-light-by-the-cone-cells-of-the-retina-briefly-describe-how-different-types-of-cone-cells-give-rise-to-color-vision-what-is-the-chemical-substance-primarily-responsible-for-human-vision-how-can-its-color-absor.html
[14] https://www.sciencedirect.com/science/article/abs/pii/S1350946221001014
[15] https://www.sciencedirect.com/science/article/pii/S0005272813001461
[16] http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html
[17] https://en.wikipedia.org/wiki/Cone_cell
[18] https://www.ncbi.nlm.nih.gov/books/NBK11059/
[19] https://www3.cs.stonybrook.edu/~lori/classes/ColorPerception/addproperties.html
[20] https://askabiologist.asu.edu/rods-and-cones