by Anastacia Wienecke
There is much more to color than meets the eye. Quantitative measurements like weight, volume, temperature, and length are routinely measured with ease and accuracy in everyday life. But color? Color is a subjective experience that is special to you.
How does the eye perceive color, anyway?
The back of the eye interior, called the retina, is dotted with millions of photoreceptor cells called rods and cones. “Photoreceptor” comes from the Latin words “photo”, which means light, and “receptor” which means taken back. Staying true to this name, these rods and cones receive photons coming in through the pupil, translate this information into electrical impulses, and send these impulses through the optic nerve to the brain.
Your 91 million rods are specialized for low light conditions, so when you’ve dimmed the lights and are ambling towards bed, it is your rods that keep you from tripping over the family cat! The one snag is that rods don’t detect color, so when the room gets dark enough, you might find yourself unable to differentiate between Earl, the gray cat, and Hershey, the chocolate cat.
Your 4.5 million cones work best in bright light conditions and come in three types. The only difference between types is the relative quantity of three photosensitive proteins called opsins. Cones most sensitive to red light contain high amounts of opsins that absorb long wavelengths in the visible spectrum. Cones most sensitive to green and to blue light contain high amounts of opsins specialized in detecting medium- and short-wavelengths, respectively.
Each type of opsin is built by the body using instructions stored in DNA. When all goes according to plan, all three opsin types are made faithfully and the individual perceives color as a mix of red, green, and blue. If however, a genetic mutation creates an improperly formed opsin, then the individual can no longer use that opsin to detect the corresponding range of visible light. This is how color blindness arises.
What’s special about a bird’s eye view?
Our avian friends have an extra type of cone that humans don’t have, one with opsins specialized in detecting wavelengths in the UV range. While it is difficult to imagine what exactly this might look like, we can be sure that bird vision is much more varied than our own. For instance, take a look at the Mimulus flower. On the left is a photo taken in visible light. The petals appear homogeneously yellow minus a few speckles here and there. On the right is a photo of the same flower taken in UV light. We see that parts of the flower strongly absorb the UV light (and so appear in black), whereas other portions of the flower strongly reflect UV light back to the observer (and so appear lighter). We cannot see these differences, but birds and butterflies, who are reliant on the nectar these flowers produce, can recognize these patterns as clear as day. They perceive color in a vastly different way than we do.
Biofluorescence: the Effects of UV Light We Do See
While we do miss more than a few visual cues due to our eyes’ confinement to the visible spectrum (light with wavelengths between 350 and 750nm), we can see the effects of UV light hitting biofluorescent animals. UV light is high-energy, and when it hits certain photosensitive pigments, a characteristic glow is produced as low-energy light is radiated outwards. This radiated light is visible to our eyes. Among others, the platypus, springhare, and flying squirrel all biofluorescence in different colors (see below).
Our ability to sense the world around us through color is remarkable, and new discoveries are being made daily. Color vision is a gift, so next time you have a quiet moment, try taking note of the beautiful rainbow of colors right in front of you!
Sources:
The machinery of colour vision by S. G. Solomon and Peter Lennie
True Colors: How Birds See the World by Cynthia Berger
Springhares Glow Orange and Red Under Ultraviolet Light by Natali Anderson
