Is it possible for something to not reflect any visible light (for humans)? Eg can something only reflect infrareds or ultraviolets or gamma rays etc.?
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It's an engineering problem, more or less, to develop a material that reflects no visible light. The closest we've come so far to a material that reflects no visible light at all is, afaik, Vanta-black, which absorbs 99.965% of visible light. Because it reflects almost no visible light it looks extremely black.
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$\begingroup$ One more question will vanta-black emit infrared light at, say, 20°C? $\endgroup$ May 8, 2018 at 16:56
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$\begingroup$ Any body (above absolute 0) will emit electro-magnetic radiation in the form of black body radiation. That is not something that can be engineered away. See here: en.wikipedia.org/wiki/Black-body_radiation $\endgroup$– enumarisMay 8, 2018 at 17:08
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$\begingroup$ @user1509229 There is a substantial difference between emitting infrared and reflecting infrared. And also between near infrared and far infrared. Near infrared is just like reflected light, except that we can see it only in night vision goggles using infrared flashlights. In contrast, far infrared is emitted by heated bodies, is weaker and requires more sensitive (military grade) goggles. $\endgroup$ May 8, 2018 at 20:47
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$\begingroup$ @safesphere So like, what does a heated body emits? Just far infrared? Or does it also emits near infrared? $\endgroup$ May 11, 2018 at 14:32
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$\begingroup$ @user1509229 Conceptually any heated body emits all kind of radiation, but most of it is centered around the peak that depends on the temperature. The Sun is a heated body that emits mostly visible light, but also UV and IR. A human body emits mostly far infrared, but reflects visible light and near infrared. Does a human body emit near infrared? Well, if it did, it also would emit some visible light that is "near" in the spectrum. Do we glow in the dark like a heated metal? Not really. Not enough to see, but a heated metal that glows in the dark does emit near infrared. $\endgroup$ May 11, 2018 at 14:47
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As far as I know there is no material that fully reflects all the frequencies of waves found in the electromagnetic spectrum, as energy is indeed minimally absorbed in all materials. On the other hand, a black hole which is almost perfect black body (Max’s theory) absorbs every single type of electromagnetic wave as light cannot displace under such intense forces. The blackbody absorbs any radiation ( here light) falling on it and appears black under reflection when illuminated from outside. When this blackbody is heated it radiates electromagnetic energy due to the thermal agitation of Electrons on it's surface.
