Amateur level. Popular Science books usually start describing the double-slit experiment in terms of waves, by showing a drawing in which waves are pictured as concentrical circles emerging from the source. When passing through the slits, new concentrical circles are drawn emerging from each slit. When the circles reach the screen it's easy to understand the building up of interference. Then they go on saying that you may tune the source beam so that it emits a single particle (eg an electron) at a time. This gives me a problem for understanding the wave picture. A concentrical circle expands from the source point or slit towards every direction (the circle is a 2-dimension drawing but obviously it represents concentrical spheres). But when we consider a single particle, obviously it can not travel to any and every direction. When we think of the beam firing a single particle at a time, and we want to use the wave picture, can we think of it as a spherical wave expanding in all directions? this would seem weird! Or do we have to think of it as an ondulation traveling along a single line direction as we do when we use the point-particle picture? But if we think of the wave as an ondulation along a single line, it becomes difficult to see how interference would build up, doesn't it? Thanks!