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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

Qmechanic
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meee
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2 Answers2

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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.

BowlOfRed
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  • 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.... – meee May 07 '18 at 18:18
  • @meee please read my answer – anna v May 07 '18 at 18:24
  • yeah but only 45% is reflected back....then obviously the object is mostly transparent right....correct me if am wrong....tnq – meee May 16 '18 at 06:39
  • 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? – BowlOfRed May 16 '18 at 06:57
  • 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 – meee May 17 '18 at 05:27
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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.

anna v
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  • can you tell me how it happens...... – meee May 07 '18 at 18:24
  • in the atomic level – meee May 07 '18 at 18:34
  • https://en.wikipedia.org/wiki/Rayleigh_scattering . it is like the feynman diagrams for compton scattering but the energy and phases do not change . – anna v May 07 '18 at 18:41
  • will read it and come back...thanks for the answer – meee May 07 '18 at 18:43
  • will read it and come back...thanks for the answer – meee May 07 '18 at 18:44
  • also found this https://www.sciencedirect.com/science/article/pii/S0969806X99002868 – anna v May 07 '18 at 18:46
  • the wikipedia article say "rayleigh scattering happens when light passes through transparent materials"...how a brick wall is transparent – meee May 10 '18 at 09:56
  • In optical frequencies elastic scattering causes this difusion of light, called Raleigh scattering, but the theta 180degrees elastic scatter is also there. with a wall it is only the backwards . I am trying to say that there exists elastic photon scattering, depending on the boundary conditions . The photon does not have to be absorbed, it can back scatter – anna v May 10 '18 at 10:35
  • see also my answer here https://physics.stackexchange.com/questions/339885/do-photons-bounce-at-mirrors/339901#339901 – anna v May 10 '18 at 10:46
  • is there a way to find the intensity of light scattered in specific directions – meee May 11 '18 at 06:39
  • th e smart thing is to use classical optics, because it works for optical frequencies. for infrared the wavelengths are such that any individual scatter is a difusion away from the wall, taken a photon at a time. – anna v May 11 '18 at 15:23