After my post about astronomical images on Tuesday, two separate people brought up an interesting idea: that Impressionist painter Claude Monet could see some ultraviolet wavelengths of light after the lens in his right eye was removed to save his vision from cataracts. My friend and colleague Cedar Riener and science-writing ninja Carl Zimmer independently brought up Monet’s possible extraordinary ability; Joe Hanson, inspired by Zimmer’s piece, added quite a bit of interesting detail.

I don’t know much about physiology or the psychology of color perception, so I’ll stick to broad outlines. Similarly, I’m no art scholar or historian, though I love Monet’s painting and have a special fondness for his late-period work, when the world didn’t seem to provide sufficiently large canvasses for his increasingly abstract art. If I make mistakes in any of these areas, I (as always) ask for correction and forgiveness! Read both Zimmer’s and Hanson’s pieces if you want that part of the story.

When he was 82 years old, Claude Monet suffered from such severe cataracts that he agreed to have the lens removed from his right eye. Cataracts that occur in elderly people turn the lens of the eye cloudy and yellowed, much like old glass can become discolored. The yellowing of the lens works as a filter, reducing the amount of blue light that reaches the retina. However, normal, healthy lenses filter out ultraviolet (UV) light, so when Monet’s lens was removed, he not only could see the blue hues again, he could also see a limited amount of UV—which he attempted to paint, as the images below demonstrate.

Visible light is a term relative to us humans: the combination of our crystalline lens and especially the cone cells on our retinas dictate what wavelengths of light we can perceive. That range of colors is relatively small compared to the entire spectrum of light, encompassing wavelengths from roughly 400 to 700 nanometers (0.0000004 to 0.0000007 meter), where the short wavelengths correspond to violet light and the longer wavelengths are red light. An average human can see light across these colors, but as the image at right shows, response by the cone cells isn’t perfect: some colors will show through more strongly than others. The brain takes electric signals produced by the cone cells and constructs the colors we perceive by combining the signals with their appropriate strengths. Note also that a little bit of UV light can sneak in, assuming the lens doesn’t block it; perhaps Monet’s retina was more sensitive to UV than average, though that sort of thing is hard to demonstrate.

Human evolution has obviously selected for seeing this way; other animals don’t have equivalent vision. Bees can see UV light but have less sensitivity to red light; some flower species have patterns only visible in the UV, indicating a certain amount of coevolution. (After all, it takes “effort” in a certain sense to make patterns; if that effort isn’t rewarded by reproductive success, then the strain of plant bearing the pattern won’t make as many baby plants.) Some species have more color receptors than humans do, and others have fewer. However, the range of vision in most species lingers around visible light, and that’s due to our Sun and Earth’s atmosphere.

As I mentioned in earlier posts on the subject, the color of a star is dictated by its surface temperature. Hot stars appear blue or blue-white, while cool stars appear red. However, no matter how hot or cool the star, it produces a lot of light in the visible light range, to the extent where most of a star’s light may actually be visible. In other words, visible light also happens to overlap largely with starlight! Even more, Earth’s atmosphere is mostly transparent to visible light, but blocks a lot of the UV and infrared light, meaning there is less sunlight available for vision in those wavelengths. Of course, other things can make infrared light than the Sun: humans are mammals, and our body temperature means we produce infrared light that we perceive as body heat. Some predators hunt by seeking infrared signals, even though they may not “see” those wavelengths.

Similarly, anyone with a black light knows that many objects respond to UV. A black light produces UV photons, and when they strike certain surfaces, those materials fluoresce, producing visible light. If Monet truly could see UV, then he would be seeing the photons that reflect rather than fluoresce, including in the pigments of his paints. And that’s an interesting thought (to me at least): even Monet’s ultraviolet painting depicted above likely contains colors most of us simply cannot see.

How can we describe colors? We assign names to blues and reds; label gradations and mixtures from blandest taupe to richest indigo; describe depths of color in terms of saturation and lushness. Yet we can never be certain that anyone else perceives color the same way we do. I look out the window and see gray sky, but while another person receives the same photons, the processing by cone cell and brain cell is not the same in everyone. We assume, since we cannot operate otherwise, that our shared language describes the same sights, but we cannot guarantee this assumption is actually true.

Monet’s ultraviolet vision highlights this inherent subjectivity: he painted what he saw (and the Impressionists argued that their art was the most “objective”, since they attempted to paint without details missed by the eye), but all art is translation. We do not see the UV light in Monet’s painting because our eyes cannot receive it … yet those pigments were obviously important to the painter himself. When we think about the way in which astronomical images are processed, the equipment acts as an interpretor, just as we interpret art—and the colors of Nature.