Infrared is a very large part of the electromagnet spectrum (nestled right below visible light and above radio). Just like light we're, it can bounce off of objects and rebound into our eyes. Our eyes can't see it, but if they could: it would be used exactly the same way. So infrared goggles would "pick up" everything. But what they're good at is identifying heat sources, and that's what you're thinking of when you're wondering what they'll work on.
Almost everything we see (visible light) is the result of a light source bouncing off an object. Sunlight or artificial light travel from their source, hit the surface of whatever, and then rebound into our eyes we're they're absorbed and converted into electrical chemical signals that travel to our brain. As I said above: infrared would do this too, if we could develop photo-receptors that were infrared sensitive. However, in the infrared spectrum, there are a lot more things that can act as light producers rather than just reflectors. If you look around your room now, you'll notice points of high intensity light. Probably a light bulb. It's high intensity because it's producing the light, and not just reflecting some portion of light produced elsewhere.
Well everything at temperatures you're likely to experience radiates in infrared. So everything is a light source. And infrared is huge: it's not like it's just purple, or a shade of red: if you tried to map it onto the visible color spectrum it would take up the whole thing. Meaning that your goggles would need to find a way to transform infrared information into visible light information. Either entirely eliminating the visible spectrum, or mapping over it somehow.
Speaking as someone who has actually used an thermal scope (as well as traditional NVGs):
Thermals are set up specifically to identify heat sources and *only* show the IR spectrum. (as opposed to NVGs, which more or less amplify high IR as well as visable light) You can actually change the colors used, but they use various shades of a single color to represent heat. I prefer blue, others use orange, but essentially the brighter blue (or almost white) something is, the stronger the heat source. Plants are visable, and typically have a dim glow. This may have something to do with it being night time, though? Other things are completely invisible, like glass for instance. Someone wearing glasses looks like they have 2 holes in their face - apparently glass does a very good job of blocking IR radiation, or at least confuses our equipment. Cigarettes and vehicle engines are bright white glows.
NVGs, on the other hand, show the entire spectrum, but amplified. They're not powerful enough to see body heat with, but any light close to visible light is visible. We take advantage of this by using IR laser sights and IR floodlights, and some things which emit little visible light but tons of IR light (again, cigarettes) show up very brightly. Stars are another very cool (and underdocumented) feature. With NVGs on you can see about 5x as many stars and you can without. It's stunningly beautiful the first time you look up.
For some reason, NVGs are always a shade of green. I'm not sure why.
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