I was thinking about the wave PDE: Utt-c^2*(Uxx+Uyy+Uzz) = 0 And wanted to see in respect to this PDE what is a source of light or a laser. My question is, does the light waev front always expands like a 3D sphere? Or is a light wave actually a straight line in the 3D space? If the first is true, then it means that even when you have a laser, the light of the laser should also expand to all the direction from the laser line. Maybe it will very little, but still it would expand in all the direction. If the lightwave is just a straight line, then a light source like a flash light is just a lot of straight lines coming out in a lot of directions? Why did I mention the PDE... Becaue if we look at: Utt-c^2*(Uxx+Uyy)=0 Then a certain point in (x, y, t) is affected by a cone with its axis vertical to the XY plane. So even if you have a straight line in XY with width larger then 0 (a laser?) then a point which its XY is not on this line projection on the XY plane, will be affected by it, because the cone will "catch" the line. Maybe you could clear this to me better. Thanks in advance.