Are there any objects that are transparent to humans but opaque to cats and vice-versa? What makes an object transparent or opaque is complicated as far as I know but depends on which wavelengths the observer can see.
Since cats can see different wavelengths than us humans, are there objects that are transparent to us but opaque to them? Or on the other hand, are there any objects that are opaque to them but transparent to us?
It's clear from everyday observation that glass is also transparent for cats. But what about other objects?
 A: There are three main ways in which cat sight differs from humans:

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*cats have greater sensitivity at low light-levels, partly because of a large pupil and partly because light passes twice through the light-sensitive cells. Wikipedia estimates that they can manage in one sixth the light level required by humans.


*cats have less visual acuity (they are less able to see detail)


*the spectral distribution of the sensitivity is a bit different
Point 2 means that, despite point 1, there are cases at low light level when a human will be able to perceive fine structure where a cat perceives a uniform blur. An ant can sneak up on a cat (up to some distance) but the human will see it coming.
But point 3 is the crucial one to answer the question. The helpful answer at
https://biology.stackexchange.com/questions/34317/what-portion-of-the-electromagnetic-spectrum-do-cats-see
gives reason to believe that the spectrum which cats can see extends well into the UV region, down to around 320 nm (whereas human vision tails away quite rapidly below 400 nm). So at wavelengths around, say 340 nm, this suggests the cat vision is very much more sensitive than human. Suppose we guess 50 times more sensitive. That is, I admit, just a guess on my part. But anything above 10 times would already be a significant difference.
Now consider an object which has high transmission coefficient below 350 nm and is opaque above 350 nm. It might be called a "high-pass filter" because it is transmitting the higher frequencies. Such an object will be opaque to human vision but moderately transparent to cat vision (far from fully transparent, but not opaque). So if the alien creature is hiding behind a screen made of such a material, the human will see only a dark screen but the cat will see the alien lurking.
One can in principle extend human vision further into the UV by a surgical replacement for the lens in the eye.
To get a situation the other way round (opaque for cats, transparent to humans) is harder,
because I don't think there is a region (say infra red) where human vision is more sensitive than cat vision to overall light level. However I already gave an example where human vision is superior to cats, by noting that to "see" something usually involves the ability to distinguish that something from other things in the vicinity. So now imagine
an odd kind of alien with bioluminescent skin whose net emission is the same as the background surroundings, but the light-emitting cells are not continuous but distributed in a pattern (say honeycomb) with absorbent cells in between. There will be a distance at which such a creature is invisible to the cat but visible to the human with 20-20 vision whose eyesight can resolve the pattern of dots. This is not a case of opacity or transparency, of course, but it is in the general territory of the question.
