Propagation of electromagnetic energy

Electromagnetic (EM) energy is transmitted as waves consisting of electric and magnetic fields perpendicular to each other and to the direction of travel. Light, infrared, microwave and radio waves are parts of the EM spectrum.

Characteristics of propagation

1. Velocity. In vacuum, EM waves travel at $c = 299{,}792{,}458$ m/s. In air, the velocity is $v = c/n$, where $n$ is the refractive index, slightly greater than 1 (about 1.0003). It varies with temperature, pressure and humidity.
2. Frequency and wavelength:

$$c = f\,\lambda \quad\text{or}\quad v = f\,\lambda$$

3. Straight-line travel. In a uniform medium the wave travels in a straight line. In real air, refraction bends the path slightly.
4. Reflection, refraction, absorption and scattering. At a prism the wave is reflected back parallel to its path; fog, rain, dust and haze absorb or scatter it and reduce the range.
5. Modulation. In EDM the carrier wave is modulated in amplitude (or frequency) with a known frequency, so that the measuring wave has a known wavelength.

  Transmitter ~~~~ wave ~~~~> Reflector
              <~~~ return ~~~~
  wavelength  | lambda |   one cycle = 360 degrees

Use in distance measurement

The wave travels the double distance $2D$, which equals $N\lambda$ plus a fraction of a wavelength. Atmospheric corrections are applied to the velocity because it changes with temperature and pressure. Microwaves are not affected by poor visibility, whereas light and infrared waves are; however, light and infrared waves give more accurate results.