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How does light change color?

  1. Jan 21, 2016 #1
    I understand the basics of how vision works, but correct me if I'm wrong at any point.
    1. Electromagnetic radiation is emitted from the sun as a byproduct of fusion, in the form of photons.
    2. These photons travel a vast distance at an incomprehensible speed, and bounce around the atmosphere a bit.
    3. A photon strikes something such as a banana, which absorbs all wavelengths of light but yellow, and reflects that frequency away.
    4. That yellow light happens to find its way into my eye and onto my retina, at which point the red and green cones are stimulated by this frequency, and sends neural information to my brain where the image of a banana is created, adding other abstract, 3 dimension information like size and distance from me.
    My question is this. The sun obviously does not emit yellow light. The banana is only yellow because that's what light didn't get absorbed. What is it about the banana, physically/atomically that when light hits it, it changes the light from white light to yellow? How is the photon changed by the banana to carry this new information?

    Also, is it fair to say "a photon" or am I describing it inaccurately? Should I be using language that makes it sound more wave-like than particle-like? I feel like in the context that I'm describing the phenomena, I shouldn't be.
  2. jcsd
  3. Jan 21, 2016 #2
    The banana is being illuminated not by a single photon but by gazillions of photons with different wavelengths. ('white' sunlight)
    Depending on their wavelength some of these photons are more likely to be absorbed by the material of the banana and others are more likely to be reflected by it.
    What you eyes see is the wavelengths that were mostly reflected.
    Last edited: Jan 21, 2016
  4. Jan 21, 2016 #3


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    You must not be aware that the sun emits light at all different wavelengths. After all, it is called the "yellow sun." What we see in sunlight is light which is a mixture of different wavelengths.

    Isaac Newton showed that what appeared to be white sunlight could be dispersed into different colors by directing the sunlight through a prism.


    The banana appears to be yellow not because it changes the color of the sunlight hitting it, but because the banana peel reflects just the wavelengths of the incident sunlight which we perceive to be the color yellow.

    The phenomenon of color is how our eyes perceive different wavelengths of light. The visible spectrum is light which has wavelengths between 390 nm and 700 nm.

  5. Jan 21, 2016 #4
    Ah, I see now. I did not know that, but I still don't understand the other part of it. If the banana reflects mostly yellow wavelengths and not others, why does it do that?

    What is it about the material that decides what wavelengths are absorbed and others reflected? Bananas can have brown spots on them, why does that particular spot on the banana not reflect yellow light? What is it about that collection of matter that's different than the rest? Because different parts of the banana are chemically different, made up of different atoms. Is it the individual atoms themselves that reflect certain wavelengths and absorb others? What is it about the atom that decides it should reflect yellow?

    Also, thanks for teaching me something new.

  6. Jan 21, 2016 #5
    Yes the brown spots on the banana are chemically different from the fresh yellow part of the skin.
    Different molecules are there (due to micro organisms breaking down the original structure of the skin.) (don't worry they are not harmful bacteria, simple fungi like yeast mostly)
    These different molecules have different properties for light reflection and absorbtion, so we perceive that area as brown, darker - more light being absorbed and less reflected.

    When the mould really sets in it will produce it's own fruiting bodies, and then you will be seeing blue/white rotted areas instead of brown spots, but hey that banana has just got to go before it gets to that stage.
    Last edited: Jan 21, 2016
  7. Jan 21, 2016 #6
    But what are the properties? What characteristics about those molecules is the deciding factor in which light is reflected? Is it their size? Their constituent elements? Why do the yellow wavelengths reflect and not the red? Why is red absorbed? What is it about that matter that makes it absorb red?
  8. Jan 21, 2016 #7


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    Bananas are yellow because of a molecule called auxin. Specifically, the bonds between atoms in the molecule (see diagram below) absorb photons of certain wavelengths - notably absorbing shorter wavelengths (blue) and reflecting longer wavelengths (red, green).
    Last edited: Jan 21, 2016
  9. Jan 22, 2016 #8
    That's what I was looking for! Thank you!

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