Which type of signal follows a straight line and does not bend with the curve of the Earth?

Answer 1

No electromagnetic signal travels straight through the atmosphere, except during the

rare conditions when the atmospheric refractivity is the same at every altitude. Since

the refractivity depends on the temperature, humidity, and pressure of the air, you can

imagine how rarely that quantity is constant with altitude.

When refractivity changes with altitude, the path taken by a beam of radio or light curves

as it proceeds along its path. The exact nature of the curve depends on exactly how fast

the refractivity changes with altitude at the time. Occasionally but not often, it curves up

from a straight path. Most often, it curves down from a straight path, anywhere from slightly

to drastically. It doesn't necessarily follow the curve of the earth's surface. Most often, the

earth curves down faster than the signal does. It's possible for the signal to curve down

exactly as fast as the earth does, and its path remains parallel to the surface. Occasionally,

the signal curves down faster than the earth's curvature, and it returns to the ground some

distance from the antenna or the flashlight where it started. That situation happens more

often in desert regions, where the ground cools rapidly at night. Since the human eye and

brain believe that an object is located in the direction from which its image comes, these

atmospheric conditions in the desert lead to sightings of oases in the sky and other 'mirages'.

It's no accident that the legends of flying horses and magic carpets originated in the harshest

deserts.

As you may have noticed, this stuff fascinates me. Thanks for the question.