Does ordinary window glass have a very high transmissivity in the longwave infrared band?

• A. Yes
• B. No
• C. Sometimes
• D. Only at some wavelengths

Answer: (B)

The main idea here is how materials transmit infrared radiation depends on their vibrational modes. Ordinary window glass, a silicate-based material, has strong vibrational absorptions in the infrared, especially in the longwave (8–14 μm) region. These vibrational modes—often described as reststrahlen behavior in silicates—cause the glass to absorb rather than transmit infrared energy in that band, so its transmittance is not high. It remains relatively transparent in the visible and to some extent in the near-infrared, but in the longwave infrared it is effectively opaque. That’s why windows made of ordinary glass don’t pass longwave infrared radiation well.