My last question was about IR or infrared light's ability to go through glass, but...?

Question

...I'm still confused. I was reading about PIR, or passive infrared motion sensors on www.howstuffworks.com and how they cannot detect anything behind a window. The article states that glass is transparent to visible light, but opaque to infrared. If motion detectors are supposed to detect IR energy (heat) and IR energy can pass through glass as some people state, then how come motion detectors don't get tripped?

Another thing I read was about greenhouses, and how they work by trapping in long wave IR energy on the inside. In other words, the IR energy isn't able to go through glass.

Err, could someone please clarify this for me? Thanks!

Answer 1

Depends on the wavelength of the I.R.

Visible wavelengths are 400-700 nm or 0.4-0.7 microns.
I.R. from 0.7 microns to around 3 or so microns is mostly transmitted through glass.
I.R. at longer wavelengths than this is mostly absorbed by glass.

If a detector is sensitive only to this longer-wave I.R., it will not detect objects behind glass. However, detectors that are sensitive to "near-I.R." can detect objects behind glass.

As for you question about greenhouses:
Room- and body-temperature objects emit I.R. at wavelengths considerably longer than 3 microns, so this radiation is trapped by glass in greenhouses.
An object as hot as the sun emits mostly visible and near-I.R., so the sunlight passes through the greenhouse windows and is absorbed by objects inside the greenhouse.

Hope this helps.

Answer 2

Some wavelenghts of the IR waves form a part of the visible light. Those IR rays can pass thru glass.
Long IR waves have less energy.While entering, they use the energy to get in. With the little amount of energy left, maybe they can't come out . This is a pure guess.