Written by: Saaj Chattopadhyay
Edited by: Christina Del Greco, Will Dana, Kane York, and Madeline Barron
Illustrated by: Jacquelyn Roberts
Roses are red, violets are blue
Are they really? We might see different hues!
Recently, I was careless enough to think I lost my credit card while traveling so I ordered a new one. The customer service representative asked if I wanted an image on the card and pointed me to the large library of options. My eyes skimmed the web page and settled on the image of Claude Monet’s beautiful impressionist painting “The Artist’s Garden at Giverny,” and sure enough, two weeks later, I had a gorgeous new credit card. What caught my eye in the painting was the brilliant use of purple, one of my favorite colors, to depict the irises that covered his garden. His blending of paints ensures that the longer you stare at the painting, the more colors you see. It made me appreciate how our eyes and brain work together to project such a vibrant reality.
Color is the result of how our brains process light entering our eyes. There are two sides of the story: what type of light is entering our eye, and how our eyes perceive the collected light. Thus, to understand color, we first have to understand light.
How does the eye differentiate light?
The fundamental characteristic of light is its frequency (i.e., its energy). There is no limit to what this frequency can be. The electromagnetic spectrum includes very low frequency radio waves that we use to broadcast NPR, and also very high frequency gamma rays that emanate from nuclear decay. When light falls on an object, some of it reflects off and enters our eyes. The color of the object depends on both the frequencies of light and the relative intensity of those reflected frequencies. The collected light is then directed to the retina, a screen of nerve endings at the back of the eye.
There are two types of nerve endings that make up the retina: rods and cones. Rods are sensitive to the overall intensity of the light. In contrast, cones are selectively sensitive to the intensity of only narrow frequency bands of light. Thus, they can distinguish light into “frequency buckets” based on how fast the wave oscillates (Figure 1). The number of buckets depends on the number of different types of cones in the retina. Human eyes have three types of cones that distinguish between relatively fast, medium, and slow frequencies. These frequencies represent the three base colors—blue, green, and red. When only the cone sensitive to fast oscillations is excited, the brain interprets the light as the color blue.
Similarly, green and red are assigned to light that excites the cones sensitive to medium and slow oscillations, respectively. The cones can thus be identified as blue, green, or red on the basis of what color they are most sensitive to. As a result, the retina detects four pieces of information—how intense or bright the light is and how the light is distributed between the three base colors—and sends them to the brain for interpretation. What interests me the most is how the brain interprets light that excites multiple types of cones.
How do we build the vibrant reality around us?
For example, light that oscillates with frequency about 500 terahertz (Thz) excites both the red cone and the green cone. The brain senses this light as the color yellow. However, the brain cannot always tell the difference between a single frequency of light exciting two cones and two frequencies of light separately exciting those same two cones. In other words, we see yellow when an object reflects light with the frequency of 500 Thz back, and also when an object reflects both green and red light back. This is exploited by digital screens, like those that make up the vibrant TV displays that glow at me whenever I go shopping. Most displays rely on the RGB (red, green, and blue) additive color scheme, meaning the displays are only technically capable of making red, green, and blue. However, by controlling the relative intensities of these three frequencies of light, the display can be used to render a diversity of colors. So, while the banana waiting to be eaten on my counter (let’s be honest, it is destined to become banana bread) reflects yellow light into my eyes, if I take a picture of it on my phone and show it to you, you will perceive more or less the same image, but this time red and green light from my phone’s screen will enter your eye and trick it into seeing yellow.
The majority of the color gamut can be thought of as weighted averages between two spectrally adjacent cones. When blue and green cones or green and red cones are simultaneously excited, we get a color that corresponds to a single frequency between them. But to interpret light that excites blue and red cones, which are spectrally separate, our brain chooses to invent new colors! Thus, colors like magenta and purple that make up the gorgeous irises in Monet’s garden are just my brain’s interpretation of red and blue light mixed together (Figure 2). There is no single frequency of light that could reflect off my credit card and make me perceive purple. The brain processes the average of red and blue, which would be green, as fundamentally different from what it determines as purple. Yet, the brain does not invent colors for the mixtures of red and green, which present as hues of orange and yellow, and for the mixtures of green and blue, which present as hues of cyan and teal.
All of this makes me wonder: How could we expand the colors we can perceive? If we had a fourth cone sensitive to yellow light, would the brain be able to sense the difference between yellow and the weighted average of red and green? What would that color even be?!
Saaj Chattopadhyay is an Applied Physics Ph.D. Candidate in Professor Julie Biteen’s lab, where she studies single-molecule fluorescence microscopy near plasmonic nanoparticles. Apart from microscopy, she has an active interest in science education and research-based instructional strategies. She has gathered arguably more art supplies than one would ever need and is almost always found at her desk with a hot cup of tea.
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