2
$\begingroup$

I've recently read the following text on a Brazilian physics magazine:

In fall, tree leaves change color from green to yellow, orange and red. Green color comes from chlorophyll. In older leaves, chlorophyll production reduces and green tone goes away, allowing this way that other pigments such as carotene, of yellow-orange color, and anthocyanin, of red color, dominate leaves' colors. Such observed colors come in function of sun radiation's interaction.

As we can see in picture below, chlorophyll absorb solar radiation on blue and red regions; this way, light reflected by the leaves are missing those two tones and we can see them green. Anthocyanin, though, absorb light from blue to green. In this case, light reflected by leaves with anthocyanin show up with complementary colors, i.e., red-orange. enter image description here

For instance, caroten absorb most of blue-violet spectre.

After this excerpt, the text says that it's easy to check the blue-violet spectre absorption regarding carotene.

It says that since we see caroten as yellow-orange color, we only have to look at the complementary colors of yellow and orange, which are the opposite colors looking at the diagram:

enter image description here

Since the opposite colors for yellow and orange are violet and blue, that's the top absorption: blue-violet spectrum.

My doubt is if that's a valid method indeed. That's because if I use this same method for chlorophylle, it doesn't work quite right, I think. The text says that chlorophylle absorbs solar radiation on blue and red regions. Then, from that perspective and using the method the question's taught us, the complementary colors would be orange and green, and so we would see colors orange-green.

enter image description here

Shouldn't that orange tone be away for a green leaf? Or is that method wrong? For the method, I mean, saying that top absorption colors are complementary for the top reflected color, and they're opposite on that complementary colors diagram.

$\endgroup$
1
  • $\begingroup$ What is the meaning of the grey and black sectors on the pie charts? $\endgroup$
    – nasu
    Jan 21, 2023 at 2:35

2 Answers 2

1
+50
$\begingroup$

The method is correct, in the sense that I know where to find/expect an absorption peak in a UV-VIS spectrum simply by looking at the color of the sample I’m measuring.

The difficulty in interpretation comes in the non-linear sensitivity of the eye as a function of wavelength. The eye sees color using the so-called cone-cells. Simplistic models say you have blue, red, and green cones. If you look for sensitivity curves, you notice that the sensitivity ranges for the three cone types are very wide and strongly overlap. Also, you see that the green cones have higher sensitivity than the red ones, and the blue ones have the lowest. This implies that green dominates in vision if different colors are offered at equal intensity. It is not surprising that evolution resulted in eyes especially fit for seeing tones of green and brown, for forest inhabitants such as primates.

Camera makers adjust for this. Digital cameras deduce color by putting a R, G or B filter over each individual pixel. The other two colors are then interpolated for each pixel. All commercial consumer oriented cameras employ a Bayer pattern for the RGB filter, in the Bayer pattern there are more green pixels than red and blue. That automatically weights the green signal more closely to the sensitivity of the eye in the interpolation.

The nonlinear behavior also comes into play when mixing paint. I personally find it very difficult to estimate how much of a certain color to add when I am trying to achieve a particular color tone. This is not a problem, since trial and error will get to the desired result fast, but if I pay attention, I pretty much always initially add way more or way less than what was needed.

$\endgroup$
1
$\begingroup$

I think you have basically the right idea - the one big caveat here is that physics does not explain the reason that humans perceive colors the way we do. From a physics perspective, the "color" of a light is just another way of saying its wavelength. Physics does not explain, for instance, why we see the superposition of $420$ nm (blue) light and $700$ nm (red) light as being the same as $380$ nm (purple) light.

With all that said, the basic idea here is exactly what you are thinking: the leaf must either transmit, reflect or absorb every photon of light that passes through it, so the photons that are reflected tend to be the ones least absorbed by the leaf. In the case of chlorophyll, then, you are correct in saying that the leaf reflects a mixture of orange, green and even yellow light (and even some red and blue light - remember, the fact that the leaf preferentially absorbs those wavelengths doesn't mean it absorbs all of those wavelengths). However, when it reaches our eyes, our brain interprets that mix of colors as being essentially green, which is why we see plants the way we do. I think that a deeper answer to your question of why we perceive it that way can only be found in neurology.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.