dangngohai wrote:
The “Doppler effect” refers to the universally perceived change in the apparent pitch of a sound when that sound is approaching or receding from the listener. Specifically, whenever the distance between the listener and the source of the sound is decreasing, the sound will be perceived by the listener as higher than its true pitch; whenever that distance is increasing, the sound will be perceived by the listener as lower than its true pitch. If the distance between the listener and the source is not changing, the listener will perceive the true pitch of the sound.
If the above principle holds, which of the following should be observed as an eastward-traveling train is approached by a westward-traveling train that is blowing its horn?
(A) Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound lower than the true pitch.
(B) Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound lower than the true pitch.
(C) Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound higher than the true pitch.
(D) Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound higher than the true pitch.
(E) Passengers in both trains should hear a sound that is higher than the true pitch.
OFFICIAL EXPLANATION
Since the westbound train is blowing its horn, the westbound train is “the source of the sound.” Therefore, the passengers on the westbound train are traveling along with the source of the sound, and thus are neither approaching the source nor receding from it. Hence the westbound passengers should hear the true pitch of the sound.
According to the passage, the passengers in the eastbound train are approaching the source of the sound, the westbound train; the distance between the eastbound passengers and the sound is decreasing. Therefore, according to the given description of the Doppler effect, the eastbound passengers should perceive a sound that is higher than the true pitch.
(A) Passengers in the eastbound train do not hear the true pitch of the horn; passengers in the westbound train do.
(B) It is true that the westbound passengers hear the true pitch; the eastbound passengers, however, hear a sound that is higher than the true pitch.
(C) Passengers in the eastbound train do not hear the true pitch of the horn; passengers in the westbound train do.
(D) CORRECT. Westbound passengers do hear the true pitch of the sound. Eastbound passengers do hear a sound that is higher in pitch than the true sound.
(E) It is true that eastbound passengers hear a sound that is higher than the true pitch; the westbound passengers, however, hear the true pitch.
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