As with visible light waves, radio waves must equal a critical angle in order to be transmitted between regions of different refractive indices. When less than the critical angle is experienced, the waves are bent at the interface back into the medium from which they originate. In the case of an abrupt interface like air and water in a glass, the bending is equally abrupt. Layers of the atmosphere present a much more gradual interface so the reflection tends to exhibit a more curved shape.
This image shows the familiar pencil in a glass of water
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