Ground wave, refraction and reflection of sky waves, diffraction of…

The idea

Radio communication depends on how waves interact with the Earth and atmosphere. Ground waves travel along the surface, following its curvature by diffraction; this works best for long wavelengths, which diffract more around obstacles and the Earth. Sky waves travel upwards, refract gradually through the ionosphere as electron density changes, and can return to Earth; reflection is a useful limiting description. The ionosphere's varying ionisation means sky-wave reception changes with frequency, time of day and solar activity. Shorter wavelengths diffract less, so ground-wave range falls. A practical example is long-wave AM broadcasting reaching beyond the horizon by ground wave, while short-wave signals reach distant receivers via sky waves.

What earns the marks

• Ground waves propagate along or near the Earth's surface and can follow its curvature because long-wavelength radiation diffracts significantly around the Earth.
• Diffraction is greater when wavelength is comparable to, or larger than, the size of the obstacle or aperture; the Earth itself acts as the obstacle for ground waves.
• Sky waves travel into the atmosphere and are gradually refracted by the ionosphere, where free electrons make the refractive index vary with height and frequency.
• Reflection at an ionospheric layer is a simplified model of the continuous refraction that can return a sky wave to the ground.
• The ionosphere's ionisation varies with time of day, season and solar activity, so sky-wave propagation is variable and frequency-dependent.
• Shorter wavelengths diffract less around the Earth, so ground-wave communication is more effective at long wavelengths.
• Multiple hops between the ionosphere and the Earth's surface can allow sky waves to travel very long distances.

Common mistakes and how to put them right

• Treating ground waves as travelling in a straight line through the Earth.

  Put it right by explaining that they follow the surface and spread around the curvature by diffraction.

• Describing sky waves as reflecting perfectly from a sharp mirror-like layer.

  Put it right by explaining gradual refraction in a medium whose refractive index changes with height, with reflection as a limiting description.

• Assuming the ionosphere is constant.

  Put it right by linking its changing ionisation to time of day, solar activity and frequency, which changes which frequencies return to Earth.

Exam tips

• When explaining ground-wave range, explicitly link wavelength to the amount of diffraction around the Earth rather than saying only that long waves travel further.
• For sky waves, describe the change in refractive index with height in the ionosphere and state that the wave bends progressively until it may return to Earth.
• Use a labelled diagram showing the Earth's surface, the ionosphere and possible paths; annotate where diffraction, refraction and reflection occur.