Seeing the world through a biophysicist’s lens

Philip Nelson demonstrates how seemingly simple questions like ‘What is light?’ help scientists understand, and improve, how people visualize the world around them.

a line of test tubes filled with a rainbow of colored chemicals
A series of quantum dots, particles that are only a few nanometers in size, with the contents of each vial differing only in the physical size of the crystals, which causes them all to glow at different frequencies. This concept is similar to an organ pipe, which also produces sound at a particular frequency that is related to its size. (Image: Prof. Marija Drndić, University of Pennsylvania). 

Biophysicist Philip Nelson kicked off last week’s Science Café lecture with an unexpected physics demonstration. The lights inside the packed room at the Wilma Theater were dimmed as he turned on a set of carousel slide projectors. 

“So much of what we see is virtual,” Nelson said. “But now, I want to give you a direct, unfiltered experience.”

With the flick of a switch, a large square of red light was projected onto the wall, followed by a similarly sized block of green. By adjusting the lenses so the colors overlapped, Nelson created a “new category of experience” that many in the audience would simply call “yellow.” 

Nelson then added blue into the mix, showing how colors are combined to create new shades, such as magenta (blue plus red) and cyan (blue plus green). Combining all three squares together created a near-white, but, Nelson said, it isn’t the same as the pure light that comes from the sun, even though to the human eye it looks similar. 

He continued with a brief history of 17th-century debates on what, exactly, color was. People claimed white light was colorless, but inserting a glass prism into the light revealed a rainbow of saturated colors, with some people then proposing that the prism was simply adding colors by “staining” the light. 

“Our eyes are fantastic,” said Nelson, “yet they can’t tell the difference between synthetic yellow versus spectral yellow.” He explained that human vision appears to throw away some of the more nuanced color data, information that a prism of glass can reveal but that eyes and brains cannot. Which led Nelson to the presentation’s first big question: “What is color and how do we see it?”

two rainbow-colored spectra, one that has a complete set of colors and one that is disjointed
The spectrum of pure sunlight (top panel) versus a compact fluorescent light bulb (bottom panel). A glass prism can reveal the difference between the two, but the human eye cannot. 

Nelson talked about British scientist Thomas Young, whose ideas about light were years ahead of their time—162 years, to be precise. Young was familiar with Newton’s insight that light comes in different “flavors,” with new colors created through combinations that could be added together and then re-separated. Young went further by hypothesizing that our eyes contained a mosaic of pixels, each one only sensitive to a certain range of “flavors” of light.

Young’s brilliant, indirect argument was not directly confirmed until many years later, after the creation of the field of single-cell physiology. Nelson explained that the “favorite flavors” of any receptor cell were actually differences in the ability of one critical molecule to absorb light from various parts of the spectrum. Just as Young suggested, photoreceptors in the eye fall into three classes, each with a specific sensitivity function. 

These answers resulted in the development of computer and TV screens, said Nelson. Instead of requiring millions of dots to create millions of colors, digital screens use three different kinds of dots (red, blue, and green) where the brightness of each colored dot can be adjusted to create a spectrum of colors. “But this is old,” he said. “Can we get something better than that?”

As it turns out, other animals already have something better, and humans are not the pinnacle of the evolution of vision. Birds, including chickens, have four types of photoreceptors: red, blue, green, and ultraviolet. So what would it take to have “super-chicken” vision? 

Nelson explained that having more color receptors that are sensitive to narrower ranges of light would allow for finer color discrimination. This is the approach scientists use to visualize complex biological systems, including spectral karyotyping of cancer cells or separating the complicated tangle of neurons by creating a “brainbow.”

“What sets the limit to how sensitive our eyes are?” Nelson then asked. The application of quantum theory and understanding the concept of “lumpy” light has also allowed researchers to break new barriers, with one example involving Penn faculty showing how a single-molecule “motor’’ could be tracked in real time. These advances, explained Nelson, are all thanks to seemingly simple questions that led scientists to better understand our own “imperfect” vision and how it could be enhanced. 

the back of an audience in a dark lecture room
Apacked room of over 60 people attended last week’s science cafe at the Wilma Theater. (Photo: Julian Shendelman)

Colorful demonstrations and “super-chicken” vision aside, at the heart of Nelson’s talk was the importance of pursuing answers. He emphasized that future insights into how the world works, and how humans see that world, will only happen by asking challenging questions and working hard to resolve them.

Philip Nelson is a professor in the Department of Physics & Astronomy in the School of Arts and Sciences at the University of Pennsylvania

A previous version of his lecture is available.