No direct rule. More indirect and a conscience. 6th edition have much less rules to reference. Many things are left open ended.
In previous edition you had a negative dice pool of 1 or 3 dice (that in some cases also stacked to 6 or even 10 dice) depending on if the target was visually slightly harder to see or if the target was severely harder to see. Beyond that you had the blind fire modifier of minus 6 dice when you had no idea where the target was (for example due to target 100% behind cover, due to total darkness or if target was invisible or whatever). It always felt strange (at least for me) that you could take a shot when you had no idea where the target was, but this was RAW in 5th edition. Also, in 5th edition of you cannot see the ranged attacker then you would be suffer the Unaware of Attacker situational modifier which meant no defense test is possible.
In this edition you instead seem to take a negative dice pool modifier of 3 dice or 6 dice depending on if the target is visually slightly harder to see or if the target is visually severally harder to see (no strange stacking), but if you have no idea where your target is (because you can't see [the target] at all due to for example chemicals, flashing lights, pure force of magic etc etc) then you automatically fail any test involving visibility (this last part basically replace the "blind fire" situational modifier we had in previous edition and personally I also think it makes more sense). Also, in this edition you still get to take a defense test when attacked by a ranged attacker you cannot see.
In this edition you don't actually resist the spell as you did in previous edition. In this edition you instead take a perception test (the perception test get a higher threshold if the target have the Invisible # status effect).
What I am saying is that if a potential observer fail their perception test to 'notice' the character that currently have an invisible # effect then it is impossible for the observer to visually see the target and they would automatically fail the attack.
It's not strictly RAW, but it is also not really contradicting RAW either. And, maybe more importantly, I can't find any other reading that is better supported than this.
I am pretty sure this is how it is supposed to be resolved, but I am open to discuss alternatives.