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B Two Questions about thermal radiation...

  1. Feb 8, 2017 #1
    Okay... I have two questions regarding thermal radiation...The first one is that, it is a constant fact that heat travels from hot to cold body, and all things give out thermal radiation, So arent these two facts contradictory in the sense that "everything is heating every other thing regardless of its temperature"???
    My other question is that absorption of this radiation depends upon(alongwith other things) surface texture of the absorbing body, how so???
     
  2. jcsd
  3. Feb 8, 2017 #2

    phinds

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    Why do you think it is a contradition? If you an I stand next to each other and start handing each other money, we are each enriching the other (and making ourselves poorer) at the same time. What matters is what is the ultimate balance. My preference is that you be the richer one and give me more money than I give you. Would that be a contradiction?
     
  4. Feb 8, 2017 #3

    Drakkith

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    Staff: Mentor

    Everything is transferring energy to everything else that its thermal radiation impacts, yes. But, as phinds stated, it is the net transfer that matters. A cold object will emit radiation that is absorbed by a hot object, but the hot object emits more radiation than the cold object. The cold object thus absorbs more energy over time and will heat up while the hot object will lose energy over time and cool down.

    A simple explanation just has to do with diffuse vs specular reflection. At the small scale, a rough surface reflects radiation in many different directions, some of which is absorbed by another nearby location which is lower/higher in surface height. In addition, some of the light penetrates into the material and is reflected and scattered by irregularities before emerging back out at a completely different angle than it entered at. Some materials, like felt, are used to line the inside of telescopes because all the fibers sticking out cause the light to undergo many different reflections. Upon each reflection a portion of the light is absorbed, leading to a large percentage of the light being absorbed instead of reflected.

    In contrast, a very smooth surface acts like a mirror, reflecting the light back out at equal and opposite angle from the normal angle (normal angle is perpendicular to the surface). This effect is magnified as the angle of incidence increases and is nearly 100% when the angle approaches 90 degrees from normal (nearly parallel with the surface).

    https://en.wikipedia.org/wiki/Reflection_(physics)
     
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