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link to the pdfinfrared absorption spectroscopy graph

I am told that infrared is reflected(blocked) by wall. my argument is,how can they reflect a photon if they don't absorb it.as you know infrared is absorbed by molecule which has same vibrational frequency as the incoming infrared photon

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  • $\begingroup$ Can you provide the source for the graph in your question? $\endgroup$
    – BowlOfRed
    May 7, 2018 at 17:34
  • $\begingroup$ yes..just a second $\endgroup$
    – meee
    May 7, 2018 at 17:35
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    $\begingroup$ How thick the layer of brick on which this graph is based? $\endgroup$
    – my2cts
    May 7, 2018 at 17:39
  • $\begingroup$ they took a very minute sample and performed the experiment...mostly in millimetres $\endgroup$
    – meee
    May 7, 2018 at 17:41
  • $\begingroup$ "scientists tell etc." is too vague $\endgroup$
    – my2cts
    May 7, 2018 at 17:46

2 Answers 2

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It looks to me that your question is: If the sample shows that IR is transmitted through, why does a brick wall of the same material block IR so effectively?

The graph you have shows the percentage of IR that is transmitted through a prepared disc that contains only 2mg of powdered brick. Even the portions of the spectrum with greatest transmission are below 55%.

But the wall (and individual bricks) consist of thousands of times more material. So while the very topmost layer of the brick material might allow a fraction of IR through, the next layer, and the next, and the next all have a good shot of absorbing or scattering the light. For an actual brick that is several cm thick, the fraction transmitted through will be insignificant.

When there's a couple of millimeters of coffee in the bottom of your cup, you can see a fair amount through it. But when the thickness is increased to a couple of centimeters, it's quite opaque. The brick material is similar.

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  • $\begingroup$ brick is made of a certain combination of molecule...a molecule has discrete vibrational modes and frequency...an molecule absorbs a photon if the energy of the photon is equal to excitation energy of the molecule...obviously it a unique energy amount...a molecule can reflect if it only absorbs it..obviously it cannot absorb all frequencies...so most of the photons should pass through..why is it not happening so.... $\endgroup$
    – meee
    May 7, 2018 at 18:18
  • $\begingroup$ @meee please read my answer $\endgroup$
    – anna v
    May 7, 2018 at 18:24
  • $\begingroup$ yeah but only 45% is reflected back....then obviously the object is mostly transparent right....correct me if am wrong....tnq $\endgroup$
    – meee
    May 16, 2018 at 6:39
  • $\begingroup$ Light can be absorbed, reflected, or transmitted. Brick has very low transmittance. If 45% is reflected, then close to 55% will be absorbed. Where is your 45% figure coming from? $\endgroup$
    – BowlOfRed
    May 16, 2018 at 6:57
  • $\begingroup$ i am saying about infrared.....there is no reason for brick to absorb the whole infrared spectrum...so obviously most of the frequencies are scattered $\endgroup$
    – meee
    May 17, 2018 at 5:27
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A clarification, as the answer by BowlofRed covers the specifics.

my argument is,how can they reflect a photon if they don't absorb it.as you know infrared is absorbed by molecule which has same vibrational frequency as the incoming infrared photon

This is only part of the story of how photons interact. Absorption by certain frequencies or frequency bands is one way, but also photons can scatter elastically from the collective field of the electrons at the outer layer of the material, this is called reflection. And also compton scatter , i.e. lose some energy by kicking an electron out.

So it depends on the material. Aluminum foil is very good in reflecting infrared, I use it behind a wood stove in order to keep the wall cool and reflect the heat to the room, for example.

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  • $\begingroup$ can you tell me how it happens...... $\endgroup$
    – meee
    May 7, 2018 at 18:24
  • $\begingroup$ in the atomic level $\endgroup$
    – meee
    May 7, 2018 at 18:34
  • $\begingroup$ en.wikipedia.org/wiki/Rayleigh_scattering . it is like the feynman diagrams for compton scattering but the energy and phases do not change . $\endgroup$
    – anna v
    May 7, 2018 at 18:41
  • $\begingroup$ will read it and come back...thanks for the answer $\endgroup$
    – meee
    May 7, 2018 at 18:43
  • $\begingroup$ will read it and come back...thanks for the answer $\endgroup$
    – meee
    May 7, 2018 at 18:44

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