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Why are only certain wavelengths reflected when light strikes a wall?

  1. Jun 25, 2013 #1
    This is my first forum post so I apologize in advance if I do not provide enough detail, speak clearly enough, or violate some forum etiquette.

    I have a few questions here all relating to the same thing so hopefully this isn't worded poorly or too much.

    When light strikes a surface, the color that we see is the color of the wavelength that is not absorbed by the surface. That much I know. What I'm not sure of is what is happening at the atomic level.

    1) So what is it about a material that absorbs wavelengths so much so that we can only see one color reflected off a surface from a source which emits all wavelengths like the sun? What is going on at the atomic level?

    2) When it comes to the color white, why is it that so few of the wavelengths in the visible spectrum of light are absorbed?

    3) Are the wavelengths in the visible spectrum of light the only wavelengths that can be reflected off of something as simple or as thin as dry wall? I ask because it would seem that a light's intensity is nothing when bounced off of a painted wall, yet can be quite powerful when off of the surface of water.

    4) When it comes to all of the wavelengths in the visible spectrum being absorbed in something like a black wall, why is it black? The light itself is white, the source that it is from is likely white or some other bright color, so why does a material that absorbs all of the visible spectrum appear black?

    Any answers would be much appreciated. Thank you!
  2. jcsd
  3. Jun 25, 2013 #2

    Simon Bridge

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    Welcome to PF;
    Don't worry, we've all been new sometime and we remember what it was like.
    Under "site info", in the top menu, there is a section on the forum rules and guidelines - they are pretty straight forward.

    In simple terms - photons interact with the electrons near the surface of the material. They may do so in a number of ways - they may bounce off, or get absorbed. The detailed structure of thr material (molecular and bulk, as well as atomic) affects what exactly goes on and it can get quite intricate.

    At the atomic level, some photons shift the electrons to new energy levels.
    When the electrons de-excite, they can spread the energy through the bulk material (making it warmer) instead of just releasing a single photon. The bulk material releases the heat in a number of different ways that are not visible to the human eye.

    You should be aware that the Sun does not emit all wavelengths equally - if you google for "solar spectrum" you should be able to find a graph. Some wavelengths are emitted more strongly than others. You'll have noticed that the Sun looks distinctly yelowy for example. You should also see, from the graph, that some wavelengths are absent completely - in spectrometer pictures these show up as black lines in the general "rainbow" effect. Lastly, the sunlight gets to us through an atmosphere which scatters light a great deal, and absorbs and emits it's own wavelengths.

    It is because of the fine details of the structure of the material ... typically the material does absorb some wavelengths, it's just that a wide enough range of scattered wavelengths are in the visible spectrum. If you separate the scattered light off some white object through a spectrometer you may find dark lines at some colors. These are the bits that got absorbed.

    In principle any EM wave may be scattered off a surface - which ones depend on the surface.

    Blackness is the absence of light - if light is not scattered from an object it will be black.

    The surface does not need to absorb all visible wavelengths to be black - it just has to absorb so much that the remaining amount that is scattered is not enough to register in your visual system.
    If the ambient illumination is only one color, then the surface only needs to absorb enough of that one color.

    Bottom line is that you will only experience those colors which are bright enough to register in your visual system. For these purposes the visual system is your whole eyeball, the optic nerve, the visual cortex, and whatever processing gets done to turn the physical signals into a conscious experience of vision.

    Note: if the ambient illumination is only one color, then a white surface (a surface whose color is white in white light) will appear to be the same color as the ambient light. Colored surfaces will scatter different amounts of the ambient color - appearing a greyish version of that color.

    You'll have noticed a difference between gloss and matt colors too.
    What weve been talking about with absorbtion and scattering is the diffuse (matt) color.
    The specular (gloss) color depends on reflection and may be different from the ambient color.
    If you google those terms you'll get a more complete description.

    The rest is in how the nerve impulses from your retina translate into the conscious experience of vision ... which is a mind-body problem and, afaik, nobody has worked that one out yet.
    Last edited: Jun 25, 2013
  4. Jun 25, 2013 #3
    Please comment if i am right?

    The way i understood this is:
    1) When a electromagnetic spectrum falls on a body which is NOT a black body, then some photons are reflected back completely.
    2) in cavity radiation, when light enters the cavity, the photons those are reflected by the walls of the cavity are sooner or later absorbed by it.
  5. Jun 25, 2013 #4

    Simon Bridge

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    Please do not hijack other people's threads.
    You already have a thread here where you start out talking about blackbodies - though it is related to this one.

    This thread is about color.
    I have replied to your questions in the other thread ;)
    Last edited: Jun 25, 2013
  6. Jun 27, 2013 #5
    Thanks for the lengthy and detailed reply!

    As I suspected like you said the process of the photons being absorbed by the electrons in a surface can get quite intricate. I was hoping for a simple answer that I might be able to delve into with an idea of where I'd be going.

    When we see lights from clouds, you said that was a result of reflected light as well as emitted light, that clouds absorbed and emitted light at their own wavelengths. I was unaware a source could emit its own light so easily. I was under the impression that light would only be reflected at a wavelength of the original source, and that a cloud could never emit light but only reflect it.

    Does the size of a wave influence whether or not it will be reflected or absorbed? My father works in tunnels surveying on occasion, and he told me once about how certain wavelengths don't work when you're in certain tunnels due to their size... I don't know if that was a mistake on his part or a just a simple understanding.
  7. Jun 27, 2013 #6


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    He may have been referring to radio communications. Radio is also part of the Electro-magnetic (EM) spectrum as is light, just at a much lower frequency and hence a very much longer wavelength.
    Because of this longer wavelength, tunnels can act like waveguides. Depending on the diameter, coarseness of the walls, type of rock the tunnel is in, some tunnels may propagate radio signals better than others.

    just my thoughts, without more info from you :)

  8. Jun 27, 2013 #7
    thank you for your replies, it was really very helpful.
  9. Jun 27, 2013 #8

    Simon Bridge

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    Your question was very broad in scope. The rules for EM are quite simple, but they can produce very complex emergent behavior.

    I don't think I talked about clouds directly.

    Clouds are made of dirty water - water droplets and ice crystals condensed on dust - and can be quite complex.

    Water is basically clear but if you look for waterfall images you'll see a wide range of colors depending on how the movement of the water and the impurities in it. Clouds with lots of water look greyish and thick clouds look black ... due to the amount of sunlight reflected or scattered near the top and so never gets through. Light clouds on a fine day look like they glow white because of the sunlight that gets through ... after lots of bouncing around between water droplets.

    Generally, if you shine colored light on a cloud you get a spot the same color as the light.

    However, everything absorbs some radiation and everything warm emits infra-red.

    The wavelength affects interference and diffraction - so some waves will cancel out and others get reinforced.
    Refractive index also depends on wavelength - which is how prisms work.
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