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Thread: Heating from the sun IR vs visible light

  1. #1 Heating from the sun IR vs visible light 
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    I heard that the contribution from the visible spectrum of the sun contributes more to heating of the earth than the IR part of the spectrum.
    There must be a proses that some how lower the frequency of the visible spectrum down to IR?


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  3. #2  
    Forum Radioactive Isotope sculptor's Avatar
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    Then what?
    we all stumble around in the dark?


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    OK, let me rephrase. How can the energi of visible light heat the earth ?
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    Quote Originally Posted by bluez View Post
    OK, let me rephrase. How can the energi of visible light heat the earth ?
    You are asking an excellent and deep question, one that is very poorly answered in standard introductory textbooks.

    There are many possible ways of answering, depending on what your background is, and what insights you are looking for. I'll offer an explanation, and you can post back if you don't like it.

    At an atomistic level, keep in mind that electrons in a material are in constant random motion. By chance some of those electrons, some of the time, will be in resonance with the incident light, so energy couples to the atoms that way. There is a first subtlety that is rarely articulated, and that is that the energy from photons subsequently couples into phonons (vibrations of the whole atoms, if we can speak very loosely).

    Then there is a second subtlety: The collective interaction of many vibrating atoms causes the coupled energy -- which might be within a narrow range of values -- to spread out spectrally and spatially (this is what happens as systems try to achieve thermal equilibrium).

    That's how low-entropy (narrow spectrum) incident energy can produce high-entropy blackbody-like spectra. So "simply" heating up a lump of something by shining light on it involves rather sophisticated mechanisms!
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    OK, let me rephrase. How can the energi of visible light heat the earth ?
    The same way the radiant heat from a fire directly heats surfaces close enough to it.

    If you want to know how climate science works, you might be asking the wrong question. But here goes (if this isn't what you need to know perhaps this will clarify things for you to ask the question that really interests you).

    The full spectrum of the sun's radiation heats all surfaces on the earth that it gets to, both visible and invisible parts of the spectrum. One important feature that people overlook is just how far that radiation reaches beyond the surface into the ocean. If you've ever been diving or seen pictures of others diving you know that there is always some sunlight unless the camera is very, very deep or the water is polluted or shaded by something.

    This doesn't happen on dry land. Warmth beneath the surface of the soil has got there by conduction not by direct radiation from the sun. And soil is a pretty poor conductor of warmth anyway - that's how caves and earth covered houses maintain pretty constant temperatures.

    Then you have to remember that everything in the universe that is above absolute zero radiates - a bit. Photons are released by cold objects and surfaces much less often than by warm or hot surfaces, but everything radiates - how much any given thing radiates depends on its temperature. This is where infra red gets into the picture for climate.

    The surfaces of the soil, water and everything else radiate at various frequencies. Different IR absorbing gases in the atmosphere absorb different frequencies (to put it simply: if you want graphs of absorption bands just ask for them). Some of this radiation manages to make it all the way to the top of the atmosphere and out! purely by accident.

    Because all of these molecules are at temperatures above absolute zero, they must then release the photons acquired in this process. But they release in random directions which means that about half of what they release heads back towards the surface of the earth rather than to the top of the atmosphere where it might get radiated out to space. And those earth directed photons might be interrupted by another molecule and the up-down-sideways options come into play again, so can the upwards directed photons. This constant bouncing around of IR re-radiation is what keeps earth at a liveable temperature, rather than the 33C cooler we would have without IR absorbing molecules in the atmosphere, which would put paid to anything remotely resembling the biosphere we live with. (Maybe some other biosphere, but not this one.)

    The main role of IR is in temperature maintenance of the atmosphere rather than the direct heating of earth's surfaces the full spectrum radiation from the sun is responsible for.

    Is that what you want?
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
    "nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke
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    I was mostly thinking of what happens at a atomic level. If i illuminate my own hand with a 200mw green and a 50mw red laser i don't feel anything, yet both of them are able to pop a balloon.
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  8. #7  
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    Quote Originally Posted by bluez View Post
    I was mostly thinking of what happens at a atomic level. If i illuminate my own hand with a 200mw green and a 50mw red laser i don't feel anything, yet both of them are able to pop a balloon.
    Most people don't feel anything until you get to a bit over the power levels you cite (for me, it's roughly 300mW for green lasers before I can pass a double-blind test). Pain thresholds are higher still (a good fraction of a watt). Your skin is fairly reflective to green, so the amount of power actually absorbed by your skin is quite a bit below the incident power level.

    To pop a balloon just requires weakening the smallest possible spot. And the balloon has to be of the right color so as to absorb as much of the incident laser power as possible. Tens of milliwatts can suffice to pop a balloon if the stars align.
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    A laser and a balloon. I'd think that was more about the laser's capacity to focus the emission of photons and localise, extremely, the resultant heat, regardless of the specific wavelength. Balloons are not a big challenge to any point source - physical or radiant.
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
    "nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke
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    Forum Bachelors Degree Kerling's Avatar
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    You guys are too much into Popular science. First of all, our skin, with cells and complicated bio chemistry are hardly to be considered equal to balloons. Might i suggest the simple solution? Stand near a light and see how everything heats up.The reason why light heats us, is actually pre dominantly a classical process. Light scatters and in so gives of momentum, or takes it away, to anything. Because at these temperatures our bodies are conducting, and there are no quantum jumps. Tyre momentum transfered is what heats us. In fact if we were to shine light on a nearly perfect mirror, the cost of momentum traversal versus absorption would actually cause tubs to cool.
    In the information age ignorance is a choice.
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    Electromagnetic rays can excite and atoms. What do I mean by excite an atom? It basically means that the atoms are moving faster. Possibly turning the substance into a thiner for like a liquid or solid. The light has different packets of energy (quantum). If you have lower frequencies, you have a lower average temperature. That is what people mean by quantum.
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  12. #11  
    Forum Freshman PhysicsApple's Avatar
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    Infared light is that "red" so dim we can see. Ultraviolet rays is the "purple" light we also can see. You can simply detect these lights by using certain cams. Also we shine in the infrared spectrum. So technical we glow in the dark but we can't tell with our eyes.
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