Why is a slight shaking of the picture on a television screen noticed when a low-lying aircraft passes overhead?

Answer 1

Because as long as the airplane is nearby, you're receiving two signals from the

TV station ... the direct one that you always get, plus another one reflected from

the airplane.

-- The distances traveled by the two signals are slightly different, and the difference

between them keeps changing as the airplane moves.

-- When the difference in the two path-lengths is just right so that the peaks of

both signals arrive at the same time, they add together in your TV antenna, and

for an instant, you get a combined signal that's slightly stronger than usual.

-- When the difference in the two path-lengths is just right so that the peaks of one

signal arrive together with the valleys of the other one, they cancel together in your

TV antenna, and for an instant, you get a combined signal that's almost nothing,

just as if the TV signal wasn't there at all.

-- As long as the airplane is in a position where the direct signal and the reflection

are nearly the same strength, you're watching a TV signal that's rapidly fluttering

between extra-strong to very weak to extra-strong to very weak to extra-strong etc.