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I do know that there are at least two types of yellow light: a light of a single wavelength of ~580 nm and a combination of green light and red light. (Technically, there could be more yellow light.) And the following two figures are making me confused.

Yellow, red and green reflected (from https://slideplayer.com/slide/12783633/) Only yellow reflected (from https://slideplayer.com/slide/4611001/)

Are red and green light reflected from banana? What wavelengths of white light does a banana reflect?

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4 Answers 4

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The reflectance of solid and liquid substances usually has a broad spectrum, and bananas are no exception here. Only gases have a line spectrum.

Here is the reflectance spectrum of ripe (i.e. yellow) and unripe (i.e. green) bananas.

banana spectrum
(image from "Food chemistry - Prediction of banana color and firmness using a novel wavelengths selection method of hyperspectral imaging")

You see, ripe bananas reflect light beginning from green (~ 520 nm) through yellow, orange, red, and extending to infrared.

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    $\begingroup$ I did not expect to see an actual spectrum in the answer! Bravo! $\endgroup$ Commented Jun 22, 2021 at 10:33
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    $\begingroup$ And consideration that not all bananas (or the banana-shaped things my grocery store sometimes sells) are yellow :-( $\endgroup$
    – jamesqf
    Commented Jun 22, 2021 at 16:52
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    $\begingroup$ Wow, you just happened to have a banana spectrum in your pocket - one that you could pull out and answer this question 25 minutes after it was asked. You must have very deep pockets or very good google-foo. $\endgroup$
    – Flydog57
    Commented Jun 22, 2021 at 22:26
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    $\begingroup$ @DanielDarabos, I fully expected that. Bananas are an important scientific measurement device - used for scale in all conceivable circumstances since time immemorial. $\endgroup$
    – AnoE
    Commented Jun 23, 2021 at 6:49
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    $\begingroup$ An answer which straightens it all out very nicely. $\endgroup$ Commented Jun 23, 2021 at 13:56
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Narrow-band yellow is indeed metameric to some wavelength combinations like green/red, green/yellow/orange/red etc.. But if you see a yellow-reflecting surface, you can be sure* that its reflectance spectrum is not just a narrow-band yellow. It must be a broadband green/yellow/orange/red reflector. If you find an actual surface that has a narrow peak of reflectance at the yellow wavelength and nothing else, this surface will look dark brown, because it reflects only the tiny portion of light incident on it, compared to a broadband reflector.

So, the short answer is that the figure on the right is grossly misleading, and a banana reflects much more wavelengths than just the yellows.


*Unless it's fluorescing with a narrow-band yellow, which would be unusual

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I would like to address something the other answers do not mention, that is, what really is happening at the quantum level when we say "the banana is reflecting". When photons (I am only talking about visible wavelength light) are incident on the banana, two things can happen:

  1. absorption and re-emission. Most of the photons that get absorbed by the banana, will either heat up the banana peel (transfer their kinetic energies to the molecules vibrational, rotational, and translational energies), or will be absorbed and re-emitted (through electronic transitions), and these photons will be part of the emission spectrum we can see in the other answers. Please note that even in this case, the angle of these photons is random, regardless of the incident angle. It is very important to understand that in this case the photons cease to exist, and new photons are re-emitted.

  2. diffuse reflection.

enter image description here

https://en.wikipedia.org/wiki/Diffuse_reflection

These photons will be another part of the emission spectrum that you can see in the other answers. Please note that the angle of these photons will be random, regardless of the incident angle. It is very important to understand that in this case, the photons do not cease to exist, the same photon is reflected.

Please note that in both cases, the angle of the photons (including the ones that reach our eyes and create the picture and colors of the banana) will be random, regardless of the incident angle, this is part of the reasons (and flatness of the surface at the atomic level) why bananas are usually not shiny.

Bottom line, whatever picture we have of the banana, it is because of these two ways of photons (diffuse reflection, absorption and re-emission) that come into our eyes that create this image in our brain, and these are made up of all kinds of wavelengths (as you can see from the other answers), that combine into the perception of the yellow or green banana, in our brain (and the appearance of a usually non shiny surface).

I just wanted to clarify that the answer to your question includes this explanation when we use the words "banana is reflecting".

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In simplified terms, the left hand image is true of a pure white or chrome banana and is not correct. Right-hand image is correct, of all wavelengths reflected, 580nm reflects the most and comparatively speaking the other wavelengths are partially absorbed. The simplification of the diagrams requires you see them in conceptual terms. Technically, neither are accurate.

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  • $\begingroup$ No, 580 nm is not the peak of banana reflectance. See the answer by Thomas Fritsch for the actual spectrum. $\endgroup$
    – Ruslan
    Commented Jun 23, 2021 at 17:39

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