Why the black objects absorb more light and heat from the other color objects?

  1. Why the black objects absorb more light and heat from the other color objects?
    What is the structure of the black color?
  2. jcsd
  3. Black objects absorb more lights than other colored so they appear black ! After all, things absorb more light and heat will appear more black.
    Why they absorb more light and heat depends on the electron structure of the material.
  4. Andy Resnick

    Andy Resnick 6,145
    Science Advisor
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    Hang on... just because and object is black (highly absorptive) in the visible just not mean it is also black in the infrared. Snow is black in the IR region, for example.

    How efficient an absorber a dielectric (or any material) is depends on both microscopic bulk properties (atomic/molecular absorption) but also on surface structure- textured surfaces can appear black under some conditions.
  5. DaveC426913

    DaveC426913 16,543
    Gold Member

    Without complicating the matter too much:

    Black objects are black because almost all the light that falls on them is absorbed into the material. Little or no light is reflected back toward your eye, therefore you see black. That light that is absorbed ultmately becomes heat.

    White objects are white because almost all the light that falls on them is reflected by the material. Because all the light is reflected back toward you eye, you see white. Little light is turned into heat.

    Red objects absorb light at frequencies other than red and tend to reflect frequencies near red. They don't get as warm as objects that are black.

    The nature of an object's colour has to do with the molecules and their electrons. Electrons in different states absorb different frequencies. When we build things - and especially when we paint them - we choose materials that contain molecules with very specific reflection frequencies. For example, Phthalo green - a paint colour - has very stable molecules in it that reflect light in a very specific, predictable band of green. Lamp black - another paint colour, has molecules that absorb ALL frequencies of light.
    Last edited: Jan 24, 2008
  6. I used to design electrical enclosures and there was a white paint that had an IR emissivity above 0.9 but my boss always thought I was crazy since "white doesn't radiate".
  7. Is it correct to say, white objects do nothing with the incident EM waves, and black objects change the frequency (to IR range)?
  8. No, it's not an "all or nothing" case. White reflects maybe 60% to 90% of incident light depending on the actual pigment and the shininess of the surface. Black reflects maybe 5% to 30% for the same reasons. Light energy absorbed will be changed to IR and radiated according to the emissivity at that wavelength.
  9. What always surprises me when I think about these kinds of things is shiny black objects! How weird. They reflect a lot of light (specular reflection), but they are black (diffuse absorption). Definitely a "surface structure" effect as mentioned above, but still interesting.
  10. DaveC426913

    DaveC426913 16,543
    Gold Member

    Too simplistic and misleading.

    If a bond in a molecule absorbs a given photon of a given frequency, then that photon will be converted to kinetic energy of the bond - the bond will wiggle more. Kinetic energy = heat so the object is slightly warmer.

    If the bond is impinged upon by a photon that is not of the right frequency, the bond will not absorb it. Depending on the structure, the photon (actually, the photon wavefront, but that's another story) will either pass straight through the material (meaning the material is transparent), or it will reflect off it (meaning the material is both opaque and reflective to that frequency of light.)
  11. Is that caused by a transparent & reflective layer on top of the black layer? An example would be anodized aluminium or powdercoating on, well anything. What raw material can actually do both without any change in composition?
  12. Why is kinetic energy == heat , for every substance?
  13. I was wondering how would any substance can be black for the infrared. If a substance reflects all the infrared rays, it should be white for the IR rays. If it absorbs the IR rays, it would cause the lattice to giggle, and emit heat, which is again in IR. So in any case, it should emit IR, right?
  14. DaveC426913

    DaveC426913 16,543
    Gold Member

    Um. I am not sure exactly which frequency of light would cause the lattice to giggle.

  15. DaveC426913

    DaveC426913 16,543
    Gold Member

    That's what heat is.
  16. Andy Resnick

    Andy Resnick 6,145
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    Education Advisor

    Part of the conceptual problem may be with the concept of 'black'- it's typically associated with visible colors, not with thermal imaging.

    Absoprtion = emission, by thermodynamics. A 'black body' is a perfect emitter in addition to being a perfect absorber. If the temperature of a black body is higher than the environment, it will be brighter than the environment. If it is colder, it will appear darker than the environment.
  17. Andy Resnick

    Andy Resnick 6,145
    Science Advisor
    Education Advisor

    I disagree. Kinetic energy is a measure of the state of a system, is associated with how much work can be performed and is an exact differential. Heat, on the other hand, is the difference between the internal energy and the work, is not an exact differential, and is associated with a *process* rather than a state.
  18. Heat

    absolutely, heat is the process of energy transfer, and the kinetic energy is equal to 3/2*K*T.
  19. Let me be clear on some basics. Please correct if I am wrong.
    Energy is a measure on the ability to do work on matter. Matter can do work on other matter, which is kinetic energy (eg; a moving object). EM wave can do work on matter, hence they have energy too. Heat is a just IR wave in the EM spectra.

    In my understanding, the connection between EM wave to kinetic energy comes as follows:
    An EM wave hits an atom; cause it to do work on the atom (causing the electrons to move; EM energy to kinetic energy conversion). Then the electron emits another EM energy which is Infrared ray, which we call heat.

    My question is what property of the matter/electron caused it to emit waves in the IR frequency. Why not in some other frequency?
  20. do the black body release the energy, that it absorbs?
  21. jambaugh

    jambaugh 1,798
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    Gold Member

    I found it most useful to think of the surface color of an object as how its internal molecules couple to the electro-magnetic spectrum outside the object. "Blackness" is a kind of transparency of the surface to energy passing between the molecular vibrations (heat) and the electromagnetic vibrations we call light (and ir and radio etc.)

    You see this deep inside a wood fire where the coals glow and the ashes are dark... shine a bright flashlight and it reverses with ashes white and coals dark. You can feel it when you pick up a teflon pan off the stove and hold it in front of your face. With the dark teflon side toward your face you feel the warmth, flip it around to the metal back and you fell much less heat.

    From this mental picture you can then imagine bands of surface transparency effecting colors (and distinction between u.v., visible and i.r. reflectivity.)

    Yes but before it can be released it usually gets spread about in the kinetic energy spectrum (something called equipartition of energy) so that when visible light hits the black body it is re-released as infra-red radiation in a thermal spectrum.
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