Why Infrared Light Doesn’t Pass Through Ordinary Glass
Ever tried using a thermal camera through a window and found it didn’t work? The reason has to do with the way glass interacts with different parts of the light spectrum — and it’s more science than magic.
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1. Glass Loves Visible Light, But Not All Light
Ordinary window glass (usually soda-lime glass) is transparent to visible light, which falls in the range of about 400–700 nanometers. That’s why you can see through it.
Infrared (IR) light, however, has longer wavelengths. Near-infrared starts around 750 nanometers, and mid- to far-infrared can reach into the tens of thousands of nanometers.
2. Molecular Vibrations Do the Blocking
Glass is made mostly of silica (SiO₂). The atoms in silica are bonded in a way that makes them vibrate at certain natural frequencies. These vibrations happen to match the wavelengths of most infrared light.
When infrared hits the glass, those vibrations absorb its energy and turn it into heat instead of letting it pass through.
3. Not All Infrared Is the Same
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Near-IR (like the ~940 nm wavelength used in TV remotes) actually passes through glass fairly well — that’s why remotes still work through windows.
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Mid-IR and Far-IR (used by thermal cameras to detect body heat) are almost completely blocked by regular glass, which is why your thermal image just shows a blank pane.
4. Special Materials Can Let IR Through
Some materials — like quartz, germanium, or certain plastics — are transparent to specific IR wavelengths. These are often used for infrared lenses, scientific instruments, or specialty windows.
Bottom line: Ordinary glass is transparent to visible light but opaque to most infrared because the glass molecules resonate with IR energy, absorbing it as heat. If you want IR to pass through, you’ll need the right material, not just the right window.
Here is a motion activated security camera that is powered by battery (so no wires or cabling) that CAN record through a window both day and night!
Z82 security camera, can record through glass. Motion activated and battery powered. Low light camera that does not use infrared.
