# Why doesn't infrared radiation travel through glass

1. Jun 6, 2010

### LostConjugate

Lower frequency should have an easier time getting through glass than that of visible light.

However remote controls for electronic devices to not work when glass is in the way.

Then you go down even further into the spectrum to my wireless router and it has no problem going through glass or even wood.

What is so special about the frequency of infrared that even glass stops it?

2. Jun 6, 2010

### vandegg

Glass is designed to allow visible light to pass through it, and a lot of the time is also specifically designed to block other parts of the spectrum for efficiency. I don't think that there is anything special about IR light. It isn't sufficient to say that because one type of light has a lower frequency than another it will pass through an object. Whether em radiation goes through an object or not is based on the make-up of the object and the exact frequency of the light.

3. Jun 6, 2010

### Naty1

A quick read of these two sources:

http://en.wikipedia.org/wiki/Absorptance

http://en.wikipedia.org/wiki/Intensity_(physics [Broken])

leads me to think that your router is much more powerful...it seems the effects are more power related than frequency related, but you better check that out to be sure.

Last edited by a moderator: May 4, 2017
4. Jun 6, 2010

### Naty1

Govrnment regulations on energy star ratings somewhat thwarted my efforts at buying hte most efficient replacement windows for my home..there is a long thread about that somewhere here..but you might find this tidbit interesting:

http://en.wikipedia.org/wiki/Low-E#Low-emissivity_windows

So in fact some materials block IR...in contrast to your hypothesis.
(I just saw vandegg's post which appeared while I was typing and I agree with his comments.)

Last edited: Jun 6, 2010
5. Jun 6, 2010

### LostConjugate

thats interesting.

The way I understand it is this:

In a potential, for each cycle the amplitude is reduced, higher frequency means more cycles per second which means the amplitude is reduced more rapidly. this carries over to the equations for even a photon in QM where the energy of the photon hf is less than the potential, like glass or wood.

The wave function is of the form

$$e^{ix\sqrt{2mE}}$$

and E is negative which gives a guassian that drops faster with higher kinetic energy or frequency of the photon. If you calculate the expectation value of the position using this function outside of the glass you will find that it is far greater for lower frequency photons.

There is a reason why routers are 2.4Ghz and not 200Ghz.

I guess they have done some strange things to household glass, need to get my hands on some natural glass

Last edited: Jun 6, 2010
6. Jun 6, 2010

### stevenb

Yes, you're are correct.

Glass which is transparent to visible light is generally more transparent to near IR light. In the case of an IR remote control, there should be no problem unless there are some very specific impurities in the glass, or some type of antireflection coating.

An instructive example of this is silica glass optical fiber which (when nearly pure) has a low loss wavelength of about 1550 nm, another local window at 1310 nm and is more transparent at 800 nm than at visible wavelengths. The fact that the typical wavelengths for optical communications in silica fiber are 800 nm, 1310 nm and 1550 nm (and not a visible wavelength) is a clear indication that you are correct.

Typically, Rayleigh scattering (with a 4th power wavelength dependence) decreases as wavelength increases, then material molecular absorption will increase as wavelength increases. The result is a low low wavelength (typically) somewhere in the IR region.

It's not clear to me why you got this result. I just did a quick experiment using my TV remote through a glass milk bottle which is very thick. The remote worked perfectly.

Again, you're original thought was correct and it's not clear to me why your experiment failed to transmit the IR. It must be absorption from impurities in the glass matrix, or an antireflection coating on the glass surface.

Last edited: Jun 6, 2010
7. Jun 6, 2010

### Mu naught

I'd have to do the calculations, but is it possible the infrared was simply being mostly reflected and thus the light reaching the receiving was too weak to be detected? Try doing the experiment at different angles and see if you can get it to go through.

8. Jun 6, 2010

### LostConjugate

Well it has been this way for me since I was a kid. It all started with this stereo cabinet my dad had that had a glass door, and we always had to open the glass door to use the remote controls for every device he had in it. I would try to put the remote right up to the glass door and it never worked. He is a car mechanic and explained cars well to me but he was baffled at this stereo case and couldn't understand why it was ever manufactured with a glass door.

Presently I have a glass desk which I sometimes forget and try to use devices that are under it and have to point the remote around it. The glass desk is in front of my monitor stand and some devices are in the stand so they are under the desk.

9. Jun 6, 2010

### pgardn

Dont garage door openers also use IR?

I dont have to roll down the window and stick my hand out...
hmmmm.... Glass in cars typically have lots of crud in them also...

10. Jun 6, 2010

### LostConjugate

Only 4% is reflected from air to glass transitions. This couldn't possibly be enough to prevent detection.

11. Jun 6, 2010

### LostConjugate

I have never seen a bulb on a garage door opener, I do not think they use IR, they must use a lower frequency.

Confirmed: 300-400 MHz

Last edited: Jun 6, 2010
12. Jun 6, 2010

### pgardn

Yes indeed. Closer to the radio/TV range. I learned something. Thanks.

13. Jun 6, 2010

### Mu naught

Just curious where you get this number? It depends on the angle, the index of refraction of the glass and the wavelength of the light. You can't say it's plain 4%.

14. Jun 6, 2010

### LostConjugate

I think at a 0 degree angle. Which I have tried.

15. Jun 6, 2010

### vandegg

As stated earlier on in the thread glass is designed to block IR radiation. This is, among other things, to prevent heat from escaping your home in the winter time. Every material has a specific characteristic range of electromagnetic radiation to which it is transparent to some degree. Materials can be made to absorb or reflect certain discreet wavelengths of light regardless of intensity, and, also stated earlier, the idea that that larger frequencies are more penetrating than shorter ones is not universally true.

16. Jun 6, 2010

### mgb_phys

Those bands are because of water absorption, they are the atmospheric windows where water doesn't absorb infrared.
You can make 'dry' fiber with extremely low OH content but it's a lot more work.

Of course this simply replaces the why does glass absorb IR question with why does water absorb IR!

17. Jun 6, 2010

### stevenb

Well, the transmission wavelength windows are the combined effect of decreasing Rayleigh scattering with wavelength, the increasing glass absorption with wavelength and the OH absorption peak around 1400 nm.

So your comment is relevant of course, and water (OH absorption band in particular) could be one of the impurities that help block IR in glass. Still, water impurity can't cause a thin plane of glass to be completely opaque to an 800 nm LED in an infrared remote control. I think you would need an absorption peak to be much closer to the LED wavelength to completely block the signal over such a short distance. Consider that even pure water will pass some of the 800 nm signal, even though the best transmission for water is in the blue wavelength range.

I don't disagree with what you are saying here, but I'm not sure that you've made a good case that this is the explanation for an IR remote not going through a typical window. Consider that an 800 nm LED is at a wavelength right next to the visible wavelength range. Do you know of an impurity that can be added to glass that will block the compete IR band including 800 nm and still provide a crystal clear transparent window pane in the visible range? I'm not saying you're wrong, but you would need to provide more information if this explanation is to be accepted.

18. Jun 6, 2010

### vandegg

As I understand it there is some interaction between atoms or molecules and photons of certain energies. When light hits an atom any photons that match the specific energy of the atom it is absorbed, and the rest travels through the matter. The atom which absorbed the light gets excited and electrons move around, and eventually the energy is re re-released as something else.

This is how mass spectroscopy and all of that work. Spectral lines show up at the frequency where the light is absorbed. I don't know of any specific math that describes this phenomena, but it would be neat to know if there was.

Edit:
Yeah i actually have been looking into that and it seems like you are right about that. Glass is designed to block the lower end of the IR spectrum where heat is. I have actually read that glass does not generally block IR remote wavelengths, so i do not have any more information to provide.

19. Jun 7, 2010

### LostConjugate

A google search shows people who have had success with some devices [behind glass cabinets] and not others. No explanations though.

20. Jun 7, 2010

### alxm

That's factually false and there's no theoretical reason to believe that either. Who gave you that idea?

21. Jun 7, 2010

### LostConjugate

I explained, if there is a potential the amplitude decreases per cycle, higher frequency means more cycles, I learned in E&M class.

That is why wireless routers, phones, etc are such low frequency and go through walls, tress, etc. Where visible light does not. X-rays go through objects by damaging them, because the momentum is so high.

Take a cell phone, a flash light, and an x-ray machine, and put them in a cardboard box. The only one you will not see is the flash light, and the cell phone has far less wattage than a flashlight.

22. Jun 8, 2010

### cragar

so we are saying that IR rays get absorbed by glass and not re-emitted , maybe their energy is not specific enough that when it gets absorbed by the electrons as heat it cannot be re-emitted because the difference in electrons orbital wont accommodate
the wavelength of IR rays , And maybe radio waves can pass through because their wavelengths are to long to really interact with the atoms , But i guess radio sometimes can't go through metal either , the metal creates a faraday cage . im not sure though this could be wrong . But i mean visible light can't go through carbon but it can go through diamond , so maybe it has something to do with the bonds between the atoms and the electron configuration.

23. Jun 8, 2010

### Naty1

I have some doubts about these statements and don't have time to check right now...the second staement surely does not apply to typical traditional single pane window glass nor I would expect ionterior glass, like that in shelves or cabinetry....it sure does apply to low E glass [energy star glass windows] and likely "tinted" automobile glass.

I also wondered how this discussion might relate to optical fiber....Here are some explanations that might be of interest to participants in this discussion:

http://en.wikipedia.org/wiki/Optical_fiber#Mechanisms_of_attenuation

24. Jun 9, 2010

### Born2bwire

That assumes that the effective conductivity of the substance is frequency independent. However, by Kramers-Kronig we know that any lossy medium must be dispersive (and vice-versa, stupid causality). It goes without saying that every medium has some kind of loss and thus you should expect every medium in real life to exhibit some degree of dispersion. As such, you cannot use the behavior of the medium over a finite bandwidth as an indication of its behavior over another bandwidth. Again, this can also be seen as a consequence of Kramers-Kronig whereby we see that we need to know the, I will be loose with my choice of words here, "loss information" (imaginary part of permittivity) over all frequencies to characterize the "dispersion" (real part of permittivity) over all frequencies (this is due to the fact that the real and imaginary parts are related by Hilbert transforms I believe).

The absorption spectrum of water is a good example, as others have brought up previously. You will find that the transmission is very good over the visible regions but suffers over other bandwidths, the infrared among them.

I would also maintain that X-rays do not pass through objects by damaging them. The damaging process decidedly prevents the X-rays from transmitting as it causes absorption and scattering of the wave. It is just more or less a general property that high frequency radiation interacts less and less with matter. The physics for low frequency electromagnetic radiation, say low Terahertz and below, is different from that of the higher frequencies since the lower frequencies can be explained very well via classical electromagnetics while the higher end requires quantum field theory to some or all extent to satisfactorily describe the physics.

25. Jun 9, 2010

### LostConjugate

So that is why a 10W router can go through multiple walls but a 100W flashlight can't right?

If I understand right, when comparing higher frequencies by frequency the physics don't hold the same. In a nutshell is this due to differences in molecule resonance? For example a very high frequency will not resonate with a molecule causing little disturbance.