One light-year is the distance that light travels in vacuum
in one year. It's a unit of distance.
I am quite sure the first one is Friction, but I am not sure about the second one. Is it wind?
D.
All follow an elliptical path that has two foci, rather than a circular path.
Kepler found paths are elliptical, not circular
The basic difference is that the ordinary sources are incoherent that means that the discrete frequencies merge up to give an intermediate between the maximum and minimum frequencies. While the laser is coherent containing the single frequency with maximum amplitude. thus travelling far.