Why Are Leaves Green?
/Why does a blue sky plus yellow sun make green leaves? Well, it’s not actually because adding blue and yellow make green; but, because after removing blue and yellow, green remains…
In short…
Leaves look green because they prefer blue and red light, leaving (some) green light to reflect into our eyes and camera lenses.
A leaf looks leaf-green from photosynthesis, which absorbs blue/violet and yellow/red parts of sunlight, including some lower and higher energy portions of the spectrum outside of visible light which we cant see.
‘English Ivy’, Hedera Helix, from the same plant
Think of it like this…
Picture a jar full of sunlight that has magically settled into layers of colour - a rainbow. Shake it up, mix those colours together into pure white light, just like it came out of the Sun (before we bottled it for this imaginary experiment).
Now, using a leaf as a spoon, scoop out the sunlight into another jar. The leaf is great at scooping up the blue and yellow parts of the white light, along with purples and reds, even some invisible to us like ultraviolet and infrared - but green, less so.
As you keep scooping, the remaining light in the jar looks green. Not because the leaf doesn’t scoop any green, but because it’s just better at scooping the colours (wavelengths) on either side of what our eyes perceive to be the colour green.
That’s why leaves are green.
Hedera Helix
So Why is the Sky Blue?
Blue light prevails for most of the year because our atmosphere scatters blue light 10 times more than it scatters red light.
Plants generally use that diffused blue light to produce leaves/vegetation and then, during the long summer days, they enjoy the longer periods of more direct, yellow light.
Yellow and red light is what triggers and provides the extra energy for plants to grow more complicated forms such as flowers and then fruits.
By comparing quantities of blue vs yellow light, plants can regulate themselves to stay in rhythm with the length of the day, which keeps them in tune with the seasons.
Why Are Humans Sensitive to Green Light?
Humans do, of course, also see more than just green light.
We see all the colours of the literal rainbow, as long as we’re talking about the ‘visible light’ part of the rainbow since they also include colours we can’t see outside of the visible spectrum.
Green-yellow light is what humans and many other plant-eating or omnivorous animals have the greatest ability to analyze because this sensitivity lets animals better detect fresher, more edible parts of plants before they become more difficult to digest, such as when waxy or stringy and so less worth the effort.
New shoots and leaves contain the most nutrients in the most digestible form for a hunter-gatherer, but thousands of years of agricultural selective breeding has allowed us to focus on growing more easily digested versions of the end-products of plants (less stringy squashes, juicer fruits, etc) rather than focusing on fresh shoots and crossing your fingers hoping to find a few large, ripe-enough berries. Still, there is a huge amount of that are eaten today from bamboo shoots to beansprouts to salad leaves and even unripe flower heads are all examples of our diet’s benefit from analyzing green light with our eyeballs.
The difference between healthy, green veg and diseased or toxic lookalikes, is a critical survival skill.
