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Say I have two objects (we'll call them object $1$ and object $2$) of the same material but of a different colour, placed at the same distance away from another object (we'll call object $3$) that radiates only infrared. Will the fact that object $1$ and $2$ have a different colour cause different absorptions of infrared radiation or will they absorb the same amount of infrared. What about if object $3$ only radiates ultraviolet or other light invisible to the eye instead of infrared.

What I'm trying to ask is whether the colour of an object can affect its absorption of light beyond the visible spectrum - is it only the material that matters, and why? If the colour does matter, then what colours would absorb more and what colours would absorb less, and why.

Please note that the only variable that changes is that object $1$ and $2$ are different colours, nothing else.

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Firstly, if the objects are made of the same material but are different colours then presumably they are painted or dyed or otherwise surface-pigmented. If that is the case then, unless the paint is very thin, the absorption of IR radiation will depend more on the paint than on the underlying material.

Different paints can have very different absorption (and reflection and emissivity) in IR. Although, as a rule of thumb, a pigment that is dark coloured or black in visible light is probably more absorbent than a pigment that is white or light coloured - there is no hard and fast rule. For example, there are brands of interior paint designed to reflect IR - so a coloured IR-reflecting paint may have an IR reflectivity of ~50%, compared with ~10% for normal white paint. The same can also be said about UV absorption, reflection and emissivity.

So in conclusion, unless you know the properties of the pigment that is colouring the two objects, you can't make any reasonable predictions based solely on visible appearance.

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  • $\begingroup$ Thank you! Do you though why it is that we have such a rule of thumb? Is it because there is usually a substance in darker coloured material that increases infrared radiation absorption and how would a substance increase the absorption of infrared? $\endgroup$
    – JC12
    Oct 20, 2020 at 8:24
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    $\begingroup$ The rule of thumb works approximately for black and white materials in particular. Most white pigments use oxides of titanium, zinc or (in the old days) lead - these materials are highly reflective in the visible spectrum but also fairly reflective in the near IR band. Many black pigments, for example printers ink, use fine carbon particles that absorb visible light - they are also generally absorbent in the IR. But the rule of thumb can fail for other more exotic pigments. $\endgroup$
    – Penguino
    Oct 20, 2020 at 20:52

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