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We all know the average human eye is restricted to perceiving color in the visible spectrum (or subsets thereof). Beyond the borders of that spectrum, we have infrared and ultraviolet. I understand that objects' colors are determined by the light they absorb or reflect. So, what would happen if an object only absorbed infrared and/or ultraviolet light? Would the thing be perceived as plain, old, ordinary white, or would there be a visible eeriness to it?

In other words, if an object only ever absorbed:

1) Infrared wavelengths

2) Ultraviolet wavelengths

3) Both infrared and ultraviolet, but nothing* in between

...what would be the result to the naked eye?

*And this "nothing" means there is enough of a buffer such that the object would appear white to even the unusual human who can see snippets of the UV/IR spectra.

And, to clarify, I do not mean the object will absorb all of the IR or UV spectra. Also, we are talking about an object that is reflecting the visible light but absorbing the other stuff.

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    $\begingroup$ I don't quite see how this relates to worldbuilding. This might be a better fit e.g. on Physics, maybe with their everyday-life tag. $\endgroup$
    – user
    Commented Nov 12, 2015 at 18:31
  • $\begingroup$ If you're trying to create an object that falls into the uncanny valley, something along the lines of the Void Ship from Doctor Who might be the best bet (because these would be plain, ordinary white). Something with no detectable mass, smell, light, sound, anything emanating from it, unusually smooth, oddly sharp corners. (Or maybe it's a mirror, so it absorbs no light?) That or have it make a low, hypnotic whir/beat, like binaural beats, barely audible. It's still handwavium, but it won't be plain white. $\endgroup$
    – Midwinter Sun
    Commented Nov 12, 2015 at 18:42

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It would simply appear white.

There wouldn't be any visible eeriness because the affected spectra are not visible. If the object really did not emit or reflect anything other than visible light it would probably be glowing (or burning). It would need to emit the energy it was receiving somehow, since you've restricted emission to visible light, that's the only way it could radiate (or if not, it would burn).

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  • $\begingroup$ So you're saying the beast of handwavia will either glow or have to emit a hyper beam every now and then? But seriously, could you explain what you mean by the output of what this object absorbs? $\endgroup$
    – Jon
    Commented Nov 12, 2015 at 18:31
  • $\begingroup$ @Jon Conservation of energy. If it is sitting in the sunlight absorbing the full spectrum of the sun's radiation, it's absorbing a lot of energy. The only energy it's not absorbing is in the visible spectrum, a very narrow band. Normally objects radiate energy in the infrared, as heat. This special object, being confined to visible spectrum output, could not do that, so its internal energy would continue to grow until something gives. $\endgroup$
    – Samuel
    Commented Nov 12, 2015 at 18:40
  • $\begingroup$ Rather than white, it would probably appear black, or at least as a very dark gray. That is, if it literally does absorb ALL visible light and only emits UV or IR spectrum. If it did, in fact, reflect any visible light, it would be somewhere in the gray scale. The more visible light it emitted, the whiter it would be. $\endgroup$
    – Monty Harder
    Commented Nov 12, 2015 at 20:49
  • $\begingroup$ @MontyHarder The question and answer are both about an object that absorbs all light except visible light; it absorbs no visible light. You've got it backwards. $\endgroup$
    – Samuel
    Commented Nov 12, 2015 at 20:57
  • $\begingroup$ @Samuel Right you are. That's probably because it's impossible for me to imagine an object that doesn't radiate at least some IR. $\endgroup$
    – Monty Harder
    Commented Nov 12, 2015 at 22:12
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It would appear white or metallic.

As a beautiful example, consider these slugs of metal Germanium Lenses

To us, it looks like a mirror, reflecting all light. In the IR spectrum, its a lens. IR light passes right through, and gets bent just like visible light is bent by a magnifying glass.

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  • $\begingroup$ It's specifically not absorbing IR or visible light, so is it an example of an ultraviolet absorber? $\endgroup$
    – Samuel
    Commented Nov 12, 2015 at 18:42
  • $\begingroup$ @Samuel You actually have me stumped there! I don't know if its reflecting UV or absorbing it! All the numbers I find regarding germanium are interested in the IR region, where the desirable lens behaviors can be found. Nobody seems to care what happens to non-transmitted light other than the anti-reflection coating people. $\endgroup$
    – Cort Ammon
    Commented Nov 12, 2015 at 19:21
  • $\begingroup$ Looks like it reflects it as well as visible light. $\endgroup$
    – Samuel
    Commented Nov 12, 2015 at 19:26
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I think these would all be white, black, or transparent, depending on what it was designed to do. It could also be a mirror for visible light and a lens for UV. An object's color is determined by what light it reflects, not what light it absorbs. In other words, a green shirt is, in one sense of the word, every color but green.

Something that reflects all colors of light is white. If an object only absorbed infrared, only ultraviolet, or only infrared and ultraviolet but nothing in-between, it would appear white, because white light is a combination of all visible colors. For reference, see these flowers:

What you and I see when we look at flowers

However, insects like bees can see ultraviolet, and see these flowers completely differently:

What a bee sees, looking at the same flowers… sort of

Obviously, a bee probably sees it a little bit differently, given that our brains can't even process ultraviolet*, but this is a translation. The flowers look white to us, but they are actually creating interesting patterns for those who can see UV. I'm not entirely certain, but I think this is because certain parts of the flower reflect and other parts absorb UV, creating patterns that humans can't even see.

Now, if something only reflected ultraviolet/infrared, which I think is what you're asking about – a flower colored such that if we could see ultraviolet, we would point at the flower and call it ultraviolet – it would look black. Our hypothetical flower would absorb all colors of light and would not reflect any. This is what black is: it absorbs every visible color of light. If it allowed all other light to pass through it, it would look transparent.

Water is an example of something that reflects UV light. This is why water can actually increase your sunburn. There are also mirrors and lenses in fiber optics specifically designed to allow most light to pass through but to be opaque to UV.

*Some people can see into the UV spectrum after cataracts surgery. UV can damage your eyesight, so our eyes are designed to reflect it. After you remove that protective coating, you can sometimes see it. There is also some evidence that some people may be tetrachromats, with cones to see UV.

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This answer is more to the OP's question... The resultant object doesn't have to be strictly white. It depends on the light source that is illuminating it. While it might only absorb UV and IR, if you're shining X-Ray on it, it will still appear black.

Additionally, if your light source is only red, then the object will appear red, and the same is true for any color.

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If the object simply does not absorb any light wave, then it's white. Completely black objects absorb all light waves(this is why darker objects are hotter). So your object would be white. And this is not an irreal object! It could exist.

Note, that some animals like rat, percept ultraviolet lights. There are also people, who have no "ultraviolet filter" in their eyes, so they see the light of special readers, but their eyes are also damaged.

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I understand that objects' colors are determined by the light they absorb or reflect.

This isn't true. An object's color is determined by its context in the visual scene, and the light reflected by the object. Any answer that specifies a color is necessarily wrong, examples below.

You can say that two visual scenes that differ solely on their absorption of wavelengths outside the visible spectrum will appear identical, which answers most of your question.

Objects in a scene that do not reflect light in the visible spectrum can occlude objects that do, and can therefore have a visible impact on a scene.

Some surfaces can shift the wavelengths of light reflecting of them. If surface A is shifting some wavelength of light into the visible spectrum, and you add surface B that absorbs that wavelength, the amount of light emanating from surface A is reduced. Whether this makes A appear brighter or darker depends upon the context of the scene, but the scene can appear different.

The hue, saturation and brightness will depend upon the surrounding elements in the scene. The space in the scene due to your described object can appear black, white or glowing, in the right context.

There are many, many examples where surfaces that reflect identical light appears to be vastly different colors. These are typically called lightness illusions or color illusions.

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