In what ways are the prejudiced whites' attitudes towards blacks prove to be false or incorrect by the author?
Tom Robinson is the key to your answer. Tom is the scapegoat of the white characters. He is assumed to be guilty. Harper Lee lets us know from the first mention, however, that his is innocent. Not only is this made clear, but Lee puts her strongest character up as his defender.
The other characters, the prejudice ones, recognize his innocence during the court scene. Bob Ewell comes across as arrogant, unintelligent, and violent. Mayella is less than trustworthy, though more pitiable. And Tom is presented as being compassionate and concerned about his neighbors, not to mention calm and peaceful. The defining moment is when Atticus proves that Mayella's injury had to be caused by a right handed man, and shows that Tom's right hand is permanently damaged.
This quote from Atticus best sums up the incorrect notions of the white characters.
The witnesses for the state.have presented themselves to you gentlemen, to this court, in the cynical confidence that their testimony would not be doubted, confident that you gentlemen would go along with them on the assumption-the evil assumption-that all Negroes lie, that all Negroes are basically immoral beings, that all Negro men are not to be trusted around our women, an assumption one associates with minds of their caliber. Which, gentlemen, we know is in itself a lie as black as Tom Robinson's skin, a lie I do not have to point out to you.