Q.Explain, with a reason, why the ionosphere reflects medium-wave (AM) radio signals back towards the Earth's surface, allowing them to be received far beyond the horizon, while it allows very-high-frequency FM and television waves to pass straight through it into space.

What the ionosphere is. The ionosphere is a region of the upper atmosphere, roughly 60-1000 km above the surface, where solar radiation has stripped electrons from atoms and molecules, leaving behind a layer containing large numbers of free electrons and ions. A layer containing many free charges behaves, for an electromagnetic wave, somewhat like a conductor -- and a wave incident on it can be either reflected or transmitted, depending on its frequency.

Why AM (medium-wave) is reflected. Medium-wave AM radio signals have a comparatively LOW frequency (of order $10^6\ \text{Hz}$). At frequencies below the ionosphere's own characteristic (plasma) frequency, the free electrons in the layer can respond quickly enough to the wave's oscillation to set up currents that cancel the wave's further penetration and instead reflect it back downward, towards the Earth's surface. Because this reflection can happen at a considerable height, the reflected wave can travel far beyond the visible horizon and be received at distant ground stations -- the basis of long-distance (sky-wave) AM radio reception. …

Explain what makes the ionosphere behave differently towards a wave, then contrast AM's low frequency again …

• Saying the ionosphere 'blocks' FM/TV waves rather than correctly stating it lets them pass THROUGH -- the physical effects are opposite for the two frequency ranges. …