What can be counted as a observer effect? that would cause collapse of the wave function? e.g. Is it only the which path information or is it purely the act of observing? As in if observer effect is the cause of wave function how come we can even see the interference pattern at all, Is it because we can't actually see the wave with our eyes so it is not collapsed or is it that it really is the instruments we use that effect the outcome? Is it true that if we look at say the electron after it has past the slit it will collapse and act as if it where a particle all along and do photons also suffer from wave function collapse or do they always act as a wave as they essentially are a wave ? No calculus please as I will most likely not have a clue what it means anyway. Just a simple answer will do.