Line-of-sight propagation is a characteristic of
electromagnetic radiation or acoustic
wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be
diffracted,
refracted, reflected, or absorbed by atmosphere and obstructions with material and generally cannot travel over the
horizon or behind obstacles. At
low frequency (below approximately 3
MHz)
radio signals travel as
ground waves, which follow the Earth's curvature due to
diffraction with the
layers of the atmosphere. This enables
AM radio signals in low-
noise environments to be received well after the transmitting
antenna has dropped below the
horizon. Additionally, frequencies between approximately 1 and 30 MHz can be reflected by the
F1/F2 Layer, thus giving radio transmissions in this range a potentially global reach (see
shortwave radio), again along multiple deflected straight lines. The effects of multiple diffraction or reflection lead to macroscopically "quasi-curved paths".