1. In case of electromagnetic waves, doppler shifts are independent of whether source moves or observer provided relative speed is the same. True or False?
Explanation: Whether the source moved or the observer would only matter if the waves required a material medium to propagate because then the relative motion between medium and observer would matter.
2. An observer is standing stationary in air moving at a speed of 2m/s. A source of frequency 20Hz is moving in the direction of wind with speed 10m/s towards the observer. What will be the frequency as heard by the observer? Speed of sound in air is 330m/s.
Explanation: As the air is moving, we can solve this question from the air’s frame of reference. From the air’s frame of reference, the observer will be going towards the observer at 2m/s and the source will be coming towards the observer at 10-2 = 8m/s.
F = f(v+v0)/(v+v8)
= 20*332/322 = 20.62Hz.
3. A train with a frequency ‘f’. What will be the frequency heard by a person sitting in the train? Speed of the train is ‘v’.
Explanation: There is no relative motion between the source and observer, thus the frequency heard by the person will be the original frequency of the whistle.