I am new to this field as will be apparent from my questions. I have a doubt in the basics that has been bugging me for days. This is about the original double slit experiment. As we all know that in the experiment, there is a double slit and a screen that displays the result of a photon (say) bombardment. My confusion is that I find it hard to understand whether the screen is an observer or not. I mean, the moment we placed a screen in front of the double slit as an act to observe which slit the photon is coming from, the probability wave should have collapsed and the photon should have chosen a single slit to get through (I am focusing only on the result; not the inference). In other words when there is no background screen, the photon will be taking both slits simultaneously; but the moment an observer steps in, which in this case is the screen, why doesn’t the wave collapse? We know that it doesn’t; so how can we conclude that an observer collapses the photon from a wave to a particle? If the screen is not an observer, why does the photon hit at only one point of the screen in case of a single slit or no slit....to hide the which-way info, why doesn't it behave as a wave. It is apparent that I am missing something basic here. Any inputs will be appreciated.