SPACE — When the moon completely covers the sun during a total solar eclipse, it’s not just the sky that changes. Your eyes change too.
On the grass in a Tennessee state park, Tracy Gregg felt the air turn cold when the afternoon light that had appeared suddenly faded because it was covered by bulbs. It was August 21, 2017, during a total solar eclipse that crossed most of the United States. He was right on track.
Above, the moon was moving into place above the sun, swallowing everything in shadow. In the brief darkness of totality, Gregg looked up for a moment from the sky to see the surrounding green grassland startlingly change. The green pastures were now striking hues of purple and lavender.
What happened? Gregg experienced what is known as the Purkinje effect. The Purkinje effect is a natural shift in color perception caused by fluctuations in light levels.
Under bright light, colors such as red and orange are more colorful to the human eye, compared to blue and green. However, under low light, reds and oranges become dark and dim, while purples, blues, and greens become brighter.
On April 8, another total solar eclipse will occur in North America. This event provides an opportunity for those lucky enough to be in the path of totality to experience the effects of Purkinje.
“I just remember the overall color going dark purple for only about seven or eight minutes,” said Gregg, a planetary scientist and eclipse enthusiast who chairs the geology department at the University at Buffalo.
“I admit to being a little nervous to take off my eclipse glasses, but when I did, I saw that the entire sky was just lavender….”
This perception of color transformation arises from the specific anatomy of our eyes. There are two types of cells that are sensitive to light in the back of the human eye, namely cone cells and rod cells.
Koni cells provide photopic vision, or the ability to see clearly and perceive color in bright spaces. Meanwhile, rod cells provide scotopic vision, or the ability to see in dim light but with far fewer colors.
Dull conditions such as twilight or the totality of a solar eclipse stimulate the eye to enter mesopic vision. In this condition, the retinal rods and cones work together.
“But rods and cones that are active together do not provide the very clear, colorful human vision in well-lit conditions that one might expect,” said Jay Neitz, a professor of ophthalmology at the University of Washington.
Instead, the result is hybrid vision in which the eye can perceive only a few colors. “The rods and cones do not have separate signaling pathways to the brain. So information from both converges on the same fibers connected to our brain…, and we end up with competing signals in the mesopic period.”
The human retina has three types of cones that are sensitive to color: red, green, and blue. The combination of these cones allows us to see all the colors of the rainbow during the day.
2024-03-29 04:27:14
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