when I'm studying Op-amp amplifier , in the derivations we always take voltage difference between inverting input and non inverting input is approximately zero . But I can'f find any explanation "why" ? This consideration is always taken for both inverting amplifier and non inverting amplifier . 

...
...
Why this voltage difference always remain approximately zero ? any explanation ? 

I found a lecture of my senior's  professor's where he said that "as ideal op-amp's input impedance infinite so it's positive terminal and negative terminal are virtually short. I mean very little voltage drop which can be neglected " 

I can't understand why very high (practical case) input impedance may cause very little voltage drop .