Why walls are blocking infrared waves? If bricks absorbs only certain infrared frequency as shown in the infrared absorption spectroscopy graph below

Question

link to the pdf

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

Answer 1

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.

Answer 2

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.