All beams diverge[1]. Try shining a laser pointer on something a few dozen meters away and compare the spot size to what it is when measured a few centimeters

All beams diverge[1]. Try shining a laser pointer on something a few dozen meters away and compare the spot size to what it is when measured a few centimeters in front of the laser. You may be able to reduce that size a little by choosing more optimal focusing parameters (i.e. playing with lenses) at the laser pointer, but you will not be able to prevent the beam from diverging. The only way to obtain a beam of EM radiation that does not diverge is to place the source an infinite distance away from you. That beam will still diverge at it's source, but by the time it reaches you the portions you are able to sample will be perfectly parallel to each other. [1]http://en.wikipedia.org/wiki/Beam_divergence http://en.wikipedia.org/wiki/Beam_divergence