How do we see color?
What color is a strawberry? Most of us would say red, but do we all see the same red? Color vision depends on our eyes and brain working together to perceive different properties of light.
We see the natural and artificial light that illuminates our world as white, although it is actually a mixture of colors that, perceived on their own, would span the visual spectrum from deep blue to deep red. You can see this when rain separates sunlight into a rainbow or a glass prism separates white light into a multi-color band. The color of light is determined by its wavelength. Longer wavelength corresponds to red light and shorter wavelength corresponds to blue light.
Strawberries and other objects reflect some wavelengths of light and absorb others. The reflected light we perceive as color. So, a strawberry is red because its surface is only reflecting the long wavelengths we see as red and absorbing the others. An object appears white when it reflects all wavelengths and black when it absorbs all wavelengths.
Vision begins when light enters the eye and the cornea and lens focus it onto the retina, a thin layer of tissue at the back of the eye that contains millions of light-sensitive cells called photoreceptors. Some photoreceptors are shaped like rods and some are shaped like cones. In each eye there are many more rods than cones - approximately 120 million rods compared to only 6 million cones. Rods and cones both contain photopigment molecules that undergo a chemical change when they absorb light. This chemical change acts like an on-switch, triggering electrical signals that are then passed from the retina to the visual parts of the brain.
Rods and cones are different in how they respond to light. Rods are more responsive to dim light, which makes them useful for night vision. Cones are more responsive to bright light, such as in the daytime when light is plentiful.
Another important difference is that all rods contain only one photopigment, while cones contain one of three different photopigments. This makes cones sensitive to long (red), medium (green), or short (blue) wavelengths of light. The presence of three types of photopigments, each sensitive to a different part of the visual spectrum, is what gives us our rich color vision.
Humans are unusual among mammals for our trichromatic vision - named for the three different types of photopigments we have. Most mammals, including dogs, have just two photopigment types. Other creatures, such as butterflies, have more than three. They may be able to see colors we can only imagine.
Most of us have a full set of the three different cone photopigments and so we share a very similar color vision experience, but because the human eye and brain together translate light into color, each of us sees colors differently. The differences may be slight. Your blue may be more blue than someone else’s, or in the case of color blindness, your red and green may be someone else’s brown.
Source: NEI (NIH)1
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Color Vision: Function of the human eye that is used in bright illumination or in daylight (at photopic intensities). Photopic vision is performed by the three types of Retinal Cone Photoreceptors with varied peak absorption wavelengths in the color spectrum (from violet to red, 400 - 700 nm).2
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- Source: NEI (NIH): nei.nih.gov/ health/ color_blindness/ facts_about
- Source: MeSH (U.S. National Library of Medicine)
- Source: NEI (NIH): nei.nih.gov/ health/ color_blindness
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