The rich green hues of Ireland’s hills might not stem from the process often described in textbooks for years. Chlorophyll, the pigment enabling plants to harness light energy, is commonly thought to reflect green light, giving plants their vibrant green appearance. However, this belief is a misconception.
A study from 2020 clarifies that chlorophyll doesn’t reflect light. Instead, it efficiently absorbs blue and red light, while green light is more likely to scatter from the leaf, likely due to structures like cell walls.
Chlorophyll still influences the green color of plants, but its role is more intricate than the traditional explanation suggests.
“Plant leaves are green because chlorophylls a and b absorb red and blue light more efficiently than green light, leading to a higher chance of green light being diffusely reflected from cell walls,” explains a team led by molecular plant biologist Olli Virtanen from the University of Turku in Finland.
“Chlorophylls do not reflect light.”
The usual explanation for the green appearance of plants is based on a straightforward principle of optics: an object’s color is determined by the light wavelengths it reflects. This holds true for flat, uniform objects like a Lego brick, where the reflection spectrum mirrors the absorption spectrum.
However, plant leaves are more complex, composed of various structures and materials. Such complexity means that the interaction with light might involve one component absorbing light and another scattering it.
The absorption of light by chlorophyll has been well understood for years. It absorbs most strongly in the violet-blue and red parts of the visible spectrum, and less so in the green range.
Contrary to popular belief, green light isn’t entirely useless for plants. Leaves absorb green wavelengths 20 to 30 percent less efficiently than red or blue light. Nonetheless, because green light penetrates deeper into leaves and plant canopies, it supports photosynthesis in lower layers where other wavelengths are less effective.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>Yet, a weaker absorption of green light does not imply that chlorophyll reflects it.
To explore this, Virtanen and his colleagues conducted experiments to examine how leaves of different colors reflect light, including green, yellow, and white leaves with varying chlorophyll levels. Yellow leaves have much less chlorophyll than green leaves, while white leaves have none.
Their findings revealed that yellow and white leaves reflected more green light than green leaves.
Green leaves reflected less than 10 percent of the green light directed at them. In contrast, yellow leaves reflected about twice as much, and white leaves reflected about 30 percent.
If chlorophyll were responsible for reflecting green light, then leaves with less or no chlorophyll should reflect less green light. The opposite result suggests another factor is at play in scattering the light.
The researchers propose that cellulose in the cell walls may be responsible for this scattering, although further research is required to verify this.
One might wonder why leaves without chlorophyll do not appear greener and why those with chlorophyll appear so vividly green despite reflecting little green light.
The answers lie in the properties of light and human vision.
White and yellow leaves reflect not only green light but also light across the spectrum. The dominant color reflected is the one we perceive. For yellow leaves, this is yellow.
For white leaves, reflection is uniform across the spectrum, akin to how a prism divides light into its component colors. When combined, these colors form white light.
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Interestingly, under normal daylight, the human eye is most sensitive to green wavelengths. Green appears brighter to us than other colors of the same intensity.
Thus, even a small amount of green light can dominate, so even though green leaves absorb most green light, the little that is scattered by other leaf structures is enough to create a vivid green appearance.
“With these data,” the researchers write, “we aim to dispel and correct the common misconception about chlorophyll reflecting green light.”
Now you understand why your four-leaf clover is green. Its mystical powers, however, remain elusive, perhaps somewhere over the rainbow.
The research findings were published in the Journal of Biological Education.
