Huge plot hole

True, the film could have been more transparent about it. (^ ^)

It's probably the most realistic way to do it. I honestly wouldn't be able to come up with another way.

I guess that's why it's called Science FICTION!

The mind believes what it sees and not see. What one wants a serum or liquid to do is produce translucency and stealth, so it would have to re-direct or diffuse light. What you are saying is we have to get rid of the light which is impossible, but we can deflect it or diffuse (lessen it), probably downward so an observer does not see or notice. I think the invisible man usually went out at night or when it was cold as part of this stealth. While the story is science-fiction, there are aspects of it which can be done even for a shorter time than shown on the screen.

Not a plot hole. The film is establishing that such a serum is possible. That is sufficient.

By your definition every sf, fantasy and horror film can be defined as having huge plot holes.

A plot hole is something that violates the world the film establishes. It is not something that violates real world laws.