This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Waves – Doppler Effect”.
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.
4. A passenger is standing on a platform. A train is arriving with speed =100m/s. It whistles at point B while the passenger is at point A. What is the frequency heard by the passenger if the whistle frequency is 100Hz? Speed of sound = 330m/s.
Explanation: We will consider the component of train speed along line AB, which is
= 100*(800/1000) = 80m/s.
Freq = 100(330)/(330-80) = 132Hz.
5. A source of sound moves towards an observer. What happens to the speed of sound in the medium?
c) Remains the same
d) Depends on speed with which source moves
Explanation: Speed of sound in the medium depends on the properties of the medium like bulk modulus and density, not on the speed of source creating the sound.
6. A source moves towards a stationary observer with a speed of 10m/s. If the frequency of the source is 10Hz what will be the wavelength of sound heard by the observer? Speed of sound = 330m/s.
Explanation: Apparent frequency F = f(330+v0)/(330+v8)
= 10(330)/(330-10) = 10.31Hz.
Wavelength = 330/F
= 32.007m = 32m.
7. A source of frequency 50Hz is moving towards an observer with a deceleration of 2m/s2. If its initial speed is 20m/s, what will be the maximum frequency heard by the observer? Initial distance between source and observer is 100m. Speed of sound = 330m/s.
Explanation: Apparent frequency only depends on velocity and not the distance between source and observer. So, maximum frequency will be heard when speed is maximum, which is at the start. F = 50(330)/(330-20)
8. A man jumps from a building with a source of sound. When he reaches halfway down, he starts the siren of frequency 100Hz. A person standing below him on the ground hears the first sound to be of frequency 120Hz. What is the height of the building? Speed of sound = 330m/s. Take g=9.81m/s2.
Explanation: We can find the speed corresponding to the apparent frequency and then use that to find the height of the building.
120 = 100(330)/(330-v) v = 330 – 3300/12 = 55m/s.
Using, v2 = 2gh, we can find the height covered by the man. The double of this value will be the height of the building as he is mid-way.
552 = 2*9.81*h h = 154m Height of building = 2h = 308m.
9. If both the source of sound and observer are moving, the apparent frequency will be different from the actual frequency. True or False?
Explanation: If relative motion between source and observer is zero there will be no change in frequency heard by the observer. Apparent frequency only varies in case of relative motion. So the given statement is false.
10. A man is moving towards a wall with a source of sound in his hand. Frequency of source = 50Hz. Speed of man = 8m/s. If the man sends one pulse of sound, what will be the frequency heard by him after sound reflects off the wall? Speed of sound = 330m/s.
Explanation: First we find frequency heard by wall.
That will be 50(330)/(330-8) = 51.24Hz.
Now, this will act as source and man will be the listener.
So, the frequency heard by the man
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