Line of sight

The term line of sight, often abbreviated as LOS, is quite easy to understand when talking about visible light: if we can see a point B from point A where we are, we have line of sight. Simply draw a line from A to B, and if nothing is in the way, we have line of sight.

Things get a bit more complicated when we are dealing with microwaves. Remember that most propagation characteristics of electromagnetic waves scale with their wavelength. This is also the case for the widening of waves as they travel. Light has a wavelength of about 0.5 micrometers, microwaves
as used in wireless networking have a wavelength of a few centimeters.

Consequently, their beams are a lot wider - they need more space, so to speak. Note that visible light beams widen just the same, and if you let them travel long enough, you can see the results despite of their short wavelength. When
pointing a well focussed laser at the moon, its beam will widen to well over 100 meters in radius by the time it reaches the surface. You can see this effect for yourself using an inexpensive laser pointer and a pair of binoculars
on a clear night. Rather than pointing at the moon, point at a distant mountain or unoccupied structure (such as a water tower). The radius of your beam will increase as the distance increases.

The line of sight that we need in order to have an optimal wireless connection from A to B is more than just a thin line - its shape is more like that of a cigar, an ellipse. Its width can be described by the concept of Fresnel zones.