Answer to Question 1
Answer: People on the ground below encounter the aircraft's shock wave, whereas passengers inside do notbecause the shock wave produced by the aircraft is not incident upon the passengers inside. Consider the analogous case of a boat on a river. People on shore may be doused by the bow wave as the boat passes, but passengers inside the boat don't encounter the bow wave at all. Or a dolphin at a sea-world show may splash onlookers while not encountering the effects of the splash.
Answer to Question 2
Answer: The Doppler effect is the perceived change in frequency produced by relative motion between a receiver and the source of a wave. If a wave source approaches a receiver, waves encounter the receiver more often and a higher frequency is perceived. If a wave source recedes, waves encounter a receiver less often and a lower frequency is perceived. In the case of sound, when a siren from a fire engine or ambulance approaches you, its pitch is higher. When the vehicle passes, moving away, the pitch is lower. In the case of light, the spin of a star can be determined by noting that one edge has a slightly different color than the opposite edge. The edge spinning toward you will be slightly blue-shifted, while the edge spinning away from you is slightly red-shifted. So the edge turning toward you is bluish, and the edge turning away from you is redish.
Deicing of Aircraft
Eugène Delacroix, Liberty Leading the People.
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