first of all if the beam passed over other beam, both are coherent then only interference can be observed.. if the two beams which are overlapped are from different sources then there will be no interference and there will be no change what the detector will detect otherwise.
If the 2 beams...
keeping a wall means to block the path of light.. i.e the photons from the source aren't reaching my eyes.. so i won't be able to see it... Can we say that in Vacuum, light once emitted from the source will travel forever to the infinite distances..? that's what which makes us see the distant...
light can travel light-years of distances.. its a EM wave which needs no medium to travel, but how can it travel for such long distances. what keeps it moving... does the oscillating magnetic fields and electric fields itself a manifestation of some force??