Which type of signals follow a straight line and do not bend with the curve of the Earth?

Microwave signals are indeed known for their ability to follow a straight line, which is an important characteristic when considering their propagation. This straight-line travel is a result of their high frequency, which can lead to a direct line-of-sight transmission. As such, microwave signals can experience significant losses if obstructed by obstacles or Earth's curvature.

In practical applications, this is crucial; for example, microwave communication systems often require relay stations to ensure that the signal can continue to travel from one point to another without interruption, especially over long distances where the Earth's curvature can obstruct the signal path. The necessity for direct line-of-sight makes microwaves particularly useful for point-to-point communications and certain types of radar systems.

While radio and satellite signals can curve due to atmospheric conditions, they do not maintain the strict straight line characteristic that microwaves exhibit. Fiber-optic transmissions, which use light through glass or plastic fibers, do not involve electromagnetic wave travel in the same context as microwaves or radio frequencies; instead, they rely on total internal reflection, which is not pertinent to the question about signal propagation relative to the Earth's curve.