The invisibility potion renders people invisible to [duh] visible light - say 4000 - 7000 Angstroms.
However, for longer wavelengths, say 8000 - 12000 A, it leaves people, and their retinas, quite opaque.
The potion has the additional, un-advertised feature of making one's retina sensitive to those wavelengths.
Kind of like the IR switch on the newer camcorders and the so-called "x-ray vision" pics that some people look at on the Web.
Donna and Theresa were "hot" enough [ah, that was a clue] to emit generous amounts of energy in those wavelengths.
I have to say that the girls weren't as attractive viewed in shades of gray and without visible suntans.
But ... Donna ... well, she made up for that. *wink*