The vivid hue of the lush green hills of Ireland may not have arisen from the mechanism many textbooks have touted for decades.
Chlorophyll, the pigments that allow plants to derive energy from light, are often said to reflect green light, making many plants an eye-catching emerald. But this is a common misconception.
According to a 2020 paper, chlorophyll does not reflect light at all. Instead, it merely absorbs blue and red light more strongly, leaving the green light more likely to scatter out of the leaf, probably from structures such as the plant's cell walls.
Chlorophyll still plays a role in determining plants' green color – but it's not doing it the way many textbooks state, and that may actually be far more interesting and complex than the accepted explanation.
"Plant leaves are green because green light is less efficiently absorbed by chlorophylls a and b than red or blue light, and therefore green light has a higher probability to become diffusely reflected from cell walls than red or blue light," writes a team led by molecular plant biologist Olli Virtanen of the University of Turku in Finland.
"Chlorophylls do not reflect light."
The common explanation for the green color seen in many plants relies on a very basic rule of optics: The color of an object is determined by the wavelength of light it reflects. For a flat, homogenous object such as a Lego brick, this is true – the reflection spectrum is essentially a mirror of the absorption spectrum.
However, a plant leaf is rather more complex than a Lego brick, made up of multiple structures and materials. Such a heterogeneous object may interact with light in a more complicated fashion, with one component absorbing light, and another doing the scattering.
The way chlorophyll absorbs light has been well understood for decades – its absorption is strongest in the violet-blue and red parts of the visible spectrum, and weakest in the green wavelength range.
Green light also isn't as useless to plants as many people assume. The researchers note that leaves absorb green wavelengths only about 20 to 30 percent less efficiently than red or blue light. Because green light penetrates deeper into leaves and plant canopies, it can help drive photosynthesis in lower layers that other wavelengths reach only weakly.
But a weaker absorption of green light doesn't mean that chlorophyll is reflecting it.
To investigate, Virtanen and his colleagues conducted a series of experiments to see how leaves of different colors reflect light – not just green leaves, but yellow and white leaves, too, which have different levels of chlorophyll. Yellow leaves have much less chlorophyll than green leaves of the same plant species; and white leaves have none at all.
The researchers measured how much light of each wavelength range was reflected by the leaves – and, fascinatingly, the yellow and white leaves reflected more green light than the green leaves.
The green leaves reflected less than 10 percent of the green light that shone upon them. The yellow leaves reflected around twice as much green light as the green leaves, while the white leaves reflected about 30 percent of the green light.
If the chlorophyll was responsible for reflecting green light, then the amount of green light reflected by leaves with less or no chlorophyll should have been lower. The fact that it wasn't tells us something else is doing the scattering.
That something else, the researchers believe, is likely to be the cellulose in the walls of the plants' cells, although further research will be needed to confirm.
You might also be asking yourself: if leaves without chlorophyll reflect more green light, why don't they appear greener? And if leaves with chlorophyll reflect so little green light, why do they appear so vividly green?
The answers to these questions concern the properties of light itself and our vision, respectively.
The white and yellow leaves don't just reflect green light more strongly, but light across the spectrum. The color that is reflected most strongly is the color that we see. For yellow leaves, that color is obviously yellow.
For white leaves, the reflectivity is even across the spectrum – think of how a prism splits light into its constituent colors. Mix all those colors back together, and you get white light.
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But under normal daylight conditions, there's something curious about the human eye. It's most sensitive to light in green wavelengths. Green appears brighter to our eyes than other wavelengths shining with the same intensity.
This means that only a little bit of green is needed for it to dominate – so, even though green leaves do absorb most green light, the little that's scattered by other structures in the leaves is sufficient for a vivid viridian color.
"With these data," the researchers write, "we seek out to falsify and correct the common misconception about chlorophyll reflecting green light."
So there you go. You now know why your four-leaf clover is green. The source of its mystical powers, however, lies somewhere over the rainbow.
The research was published in the Journal of Biological Education.