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B Are these explanations for color/light-absorption correct?

  1. Nov 8, 2018 #1
    I'm trying to see if my understanding of why an object's color helps determine why it absorbs light is accurate. Any corrections would be welcome. Be gentle, I left school 30 years ago. :)

    1. Why do black objects absorb heat more easily than white objects?

    Black objects absorb all wavelengths of visible light whereas white objects reflect all wavelengths of visible light. This light absorption causes the object to heat up.

    2. Why do black objects absorb more wavelengths of visible light than white objects?

    Black objects are composed of molecules whose atoms have protons or electrons in states that require less energy to move into higher energy states. Thus, it's easier for them to absorb energy than white objects whose molecules have atoms with protons or electrons that are already in higher energy states.

    3. What does light absorption have to do with heat?

    Heat is the average speed of atoms in a substance. Protons or electrons in higher energy states increase the speed of the atom. (really?? by vibrating more??)

    4. Does a white object truly reflect light or simply re-emit it faster than a black object?

    Photons don't bounce off of atoms. Rather, they are absorbed, raise the energy state of the protons or electrons in the atom, which either quickly drop back to their lower energy state (reflected) or retain that higher energy state for some time (absorbed). Thus, mirrors don't actually reflect light, they absorb photons and re-emit them rapidly rather than retain the energy.

    5. If a photon lacks the energy to raise a proton's or electron's energy state, what happens to it when it hits an atom?

    I do not know. :)
  2. jcsd
  3. Nov 8, 2018 #2

    Mister T

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    Why then all the questions about heat? Just below red in the electromagnetic spectrum is infrared. That's the part of the spectrum responsible (for the most part by far) the transfer of heat energy via radiation. It has little to do with the color, at least not in the way you're looking at the colors.
  4. Nov 9, 2018 #3


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  5. Nov 9, 2018 #4


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    The terms black and white are just shorthand way we actually describe the reflectivity of objects. That question is just about the use of words.

    When EM radiation hits an object, the different wavelengths of the radiation will interact differently with different substances and different surfaces. How that all works is down to Quantum Physics and there isn't a two word explanation but there is, for instance, a fairly approachable description of why metals are very reflective. Metals have a large number of electrons that move freely between the atoms and that is why they are 'good' conductors. This high conductivity also accounts for reflection at the surface; the electrons are caused to move in step with the varying EM fields of the incoming waves and they then re-emit (transmit) the waves of the same frequency back the way they came. In substances with a low conductivity, the energy of any electronic motion is absorbed and little energy is reflected. That's what we call black.

    As for the 'colours' of surfaces, the relative amount of absorption and reflection at different frequencies will cause some frequencies to be absorbed and others to be reflected. That's what we perceive as Colour.
  6. Nov 9, 2018 #5
    @scifi, If you want to learn about color and the relation with incident light on an object, I'll recommed BIllmeyer and Saltzman's Principles of Color Technology, the book has a layman focus, it explains the relation between light reflected and our human system vision to perceive colors
    Mister T warns you that visible light is just a narrow window in EM radiation inciding on objects
  7. Nov 10, 2018 #6


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    A book is great (of course) but they are expensive and people a shelf full of good books is no longer the style. for most members. Trawling through Google is the preferred way through for many people. We're all a bit dilettante about our non-specialities.
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