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A&S Associate Dean, Physics Chair Answers Common Fall Foliage Questions
With the start of autumn coming up on Sept. 22, the leaves are beginning to turn colors, exposing beautiful bright foliage for leaf peepers to enjoy over the next several weeks.
Alan Middleton is professor and chair of physics and the Associate Dean of Research and Scholarship at Syracuse University’s College of Arts & Sciences.
Prof. Middleton answers common questions related to the changing colors of leaves in the autumn and is available for interviews.
From a general science and physics perspective, what is behind the color changes that we are able to see in leaves?
Professor Middleton: “We see dramatic changes in fall leaves because of both the vibrant chemicals used by the tree to feed itself and our sensitive color vision. Most animals see only one or two types of color, while most humans see three bands of color: red, green, and blue. The sunlight that falls onto a leaf is strong in all three bands of color. In the summer, leaves have a lot of chlorophyll, a chemical that efficiently uses the energy from the blue light and red light to craft sugars to feed the tree. The green light that is not used by the chlorophyll bounces off of the leaves and reaches our eye, making the leaves look green.
“In the fall, as the tree becomes dormant, the chlorophyll goes away. What hangs around for a while are chemicals (carotenoids) that assist the chlorophyll. These chemicals absorb blue light. Then both green light and red light bounce off of the leaf. In our human vision, when red and green light combine, we perceive the color yellow, the color complementary to blue. Sometimes in the fall, the trees make an extra dose of chemicals (anthocyanins) in the fall that absorb green light. Then only red light is reflected from the leaf and our human vision tells us “that leaf is red”. Note that this whole story is for human eyes: Dogs see only two bands of color, while some other animals see ultraviolet light, so they all would perceive the changing leaves differently. Later, when these brilliant chemicals all fade from the fallen leaves, the leaves turn brown.”
Is there anything that people can do or use to give their leaf-peeping a different perspective?
Professor Middleton: “If you have a phone app that can filter colors (like Snapseed) turn on a red filter to make a black and white photo. Or you can look through a red piece of plastic. Using either method, you will see the red leaves pop out brightly against a dark sky. (This also depends on our red/green/blue human vision and the chemistry in the leaves.)
From a more personal standpoint, what do you enjoy about the fall color changes?
Professor Middleton: “Where I grew up, we had evergreen trees. I love our fall colors that remind me of when I moved here and walked my children to school with brilliant yellow, orange, and red leaves against a blue sky on chilly days. And I remember us picking up the leaves and looking at them, where you can see on a single leaf a map of the changes, with red on the edge, the last bit of green showing the chlorophyll along the veins of the leaf, and yellow in between.”
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