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Glass Opacity and Infrared Radiation
Name: Dan
Status: educator
Grade: 9-12
Country: USA
Date: Summer 2012
Question:
I am using a textbook, conceptual physics by Paul Hewitt, that states that glass is opaque to IR yet I know that my IR remote control is able to project a signal through my glass cabinet doors to my cable receiver. Yes, it is really glass and yes the remote is emitting IR. Can you tell me what I am missing in this apparent contradiction?
Replies:
Dan
Glass can be formulated to filter out specific frequencies of light.
In theatres, the lenses of Red lights filter out all frequencies except Red, Green lights filter out all frequencies except Green lights, etc. etc. etc.
Clearly, per your own experiments, the glass through which your IR signal travels (on both ends) is transparent to your particular IR frequency.
Textbook authors can make mistakes.
Typical TV remote IR signals are at 940 nm (nanometers) wavelength for a frequency of 319 THz (TeraHertz).
940 nm = 940 * (10 ^ (-9)) meters
319 THz = 319 * (10 ^ 12) Hertz
Please refer to this article for more info on TV remote control signaling processes:
http://en.wikipedia.org/wiki/Remote_control
Sincere regards,
Mike Stewart
Dan
From a materials science perspective, glass can be engineered, with dopants, to react to different spectra. The glass used in many greenhouses is transparent to short wave infrared rays (and visible light), but is opaque to long wave infrared rays. The incoming infrared is absorbed by the contents of the greenhouse. The contents reradiates, but at a lower frequency (longer wavelength). This becomes trapped energy due the spectral response of the glass. The greenhouse gets warmer, until the radiated wavelength is short enough to get through the glass. At this point, loosely speaking, the greenhouse reaches thermodynamic equilibrium. Some people think the incoming and outgoing wavelengths are the same. This would be true for reflection, but not for absorption-reradiation. The glass you are working with may not have the same spectrum as greenhouse glass, so the infrared transmits. I hope this helps!
---Nathan A. Unterman
You are missing some important points: Not all glasses are created equal. Some glasses are transparent to certain wavelengths in the infrared portion of the electromagnetic spectrum. It depends upon the glass; it depends upon the wavelengths. Place your hand near the inside of a pane of window glass. You will notice it feels warm. This means that some infrared radiation is penetrating through the pane of glass. The textbook you referred to is giving you misinformation.
Vince Calder
Hi Dan,
Transmission depends greatly on the chemistry of the glass, melting temperature, cooling time, coatings or doping materials in the glass, the incidence frequency and effective luminous intensity of radiation.
The author was probably focused on refraction and frequency only.
Purified SiO2 glasses are not IR transmittive because of strong OH absorption bands. B-Si and Si-Al and Si-Ti glasses are more generous in IR transmission. Coatings are capable of blocking substantial IR radiation.
http://www.schott.com/advanced_optics/english/download/schott_tie-35_transmittance_october_2005_en.pdf
has a downloadable paper on optical grade glasses.
Life experience indicates IR does pass through auto glass. My skin can get very hot by sitting in the sunlight of a closed up car. However, that is very low compared to no glass.
All in all, there are many factors at work for the success of your remote control and the educational focus of the author.
Hope this helps! PEHughes, Ph.D. Milford, NH
The short answer is that it depends on the composition of glass and
the wavelength of the IR. Different compositions of glass can absorb
strongly at different wavelengths, although IR transmittance in
general is not zero. Can you send the exact wording of the textbook?
Burr Zimmerman
Dear Dan,
As our electronic devices get increasingly more complicated, so do the remotes. I did a little nosing around and came up with this explanation. Although I do not know what type of remote you might have, the explanation seemed plausible.
RF extenders
Some IR remotes can send out both IR and RF signals. The RF signals aren't meant to control RF devices (in fact, they can't control them). They're meant to extend the operating range of the IR remote control from about 30 feet to about 100 feet (give or take) and allow the signal to penetrate walls and glass cabinet enclosures. The remote automatically transmits both IR and RF signals for every command. When you hook up an RF-to-IR converter (sometimes included with IR/RF remotes, sometimes sold as add-ons) on the receiving end, it receives and converts the signal back into the infrared pulses the device can understand. Now you've got an IR remote that can increase the volume on your home-theater stereo from your bedroom upstairs.
Remote controls are steadily increasing the number of devices and functions they can manage. Some universal remotes intended for home-theater components can learn commands for wirelessly controlled lights, so they will not only start a movie at the push of a button, but they'll also dim the lights for you. Full home-automation systems let you use one remote control to manage lighting, alarm systems and entertainment components by way of a receiver wired directly into your home's electrical wiring. Chances are it won't be long before you have a single remote control to manage every electronic device in your life.
I got this from HowStuff Works. Here's the address if you'd like to read more.
http://electronics.howstuffworks.com/remote-control1.htm
Hope this helps,
Martha Croll
Dan,
The bottom line is to find out two things:
1. What is the wavelength of the infrared transmitter?
2. How much light is transmitted through the glass at this wavelength?
Typical IR remote controls communicate with “near IR” around 950 nm wavelength. This wavelength is close to the long wavelength of the visible spectrum (about 700nm), but far enough away to be invisible. Glass can come in many forms with quite different transmission values at near IR wavelengths, but fortunately, measured data on many types of glass are available online. Simple pane glass (5 mm thick) lets about 70-90% of the light through at 950 nm. For a remote control, this amount is probably sufficient to allow the receiver to detect and decode the signal.
As I said, glass can come in many forms, and there are types of “heat absorbing” glass that are specifically designed to block much of the IR spectrum. Your cabinet glass is probably not of this type.
Kyle Bunch
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