I was reading about the classic Young's double slit experiment and I had a few questions regarding secondary wavelets a.k.a Huygen's wavelets. I proceed with the following questions assuming you know the experiment. The crucial point behind the explanation to the experiment is that light emerging from the original source, forms secondary wavelets at the 2 following slits. And from here on, books continue explaining things but I shall stop here for I have some questions. Q1 - The secondary wavelets do not form only at the slit, but they form everywhere. Why do we consider the secondary wavelets only formed at the slit? Why not in front of it? Why not behind it? Q2 - From the definition of secondary wavelets, I do not understand why nobody seems to discuss why they do produce a 'backlash' effect. Shouldn't the secondary wavelets travel backwards to affect the source itself and cause trouble? The books I have come across seem to skip discussing these issues. Is there something I have not understood?