Radio propagation that rely on reflection (actually refraction) of radio waves from the ionosphere. The ionosphere is a region of the atmosphere from about that contains layers of charged particles (ions) which can refract a radio wave back toward the Earth. A radio wave directed at an angle into the sky can be reflected back to Earth beyond the horizon by these layers, allowing long distance radio transmission. The F2 layer is the most important ionospheric layer for long-distance, multiple-hop HF propagation, though F1, E, and D-layers also play significant roles. The D-layer, when present during sunlight periods, causes significant amount of
Spherical Earth below the horizon). Because of their longer wavelengths, radio waves are even more susceptible to atmospheric refraction and reflection, which can cause radio and television signals to be received from towers thousands of miles away which cannot be seen with visible light. When the atmosphere is relatively well-mixed, the visual effects generally expected of a spherical Earth can be observed. For example, ships travelling on large bodies of water (such as the ocean) disappear over the horizon progressively, such that the highest part of the ship can still be seen even when lower parts cannot, proportional to distance from the