## Doppler Effect In Sound and Light Multiple Choice Questions

**Question 1. A source of sound with a frequency of 256 Hz is moving with a velocity of v towards a wall and an observer is stationary between the source and the wall. When the observer is between the source and the wall**

- He will hear beats
- He will hear no beats
- He will not get any sound
- He will get the sound of the same frequency

**Answer:** 2. He will hear no beats

**Question 2. The frequency of a progressive wave may change due to**

- Reflection
- Refraction
- Interference
- Doppler effect

**Answer:** 4. Doppler effect

**Question 3. A train blowing a whistle of frequency 1000 Hz is moving with a uniform velocity from west to east. The apparent frequency of the sound of the whistle to a stationary listener is 990 Hz. The position of the listener relative to the train is**

- On the north
- On the south
- On the east
- On the west

**Answer:** 4. On the west

**Question 4. A source of sound and a listener are moving in the same direction with the same velocity. If the actual frequency of sound is 200 Hz, the apparent frequency of the sound to the listener is**

- 200 Hz
- Less than 200 Hz
- Greater than 200 Hz
- None of these

**Answer:** 1. 200 Hz

**Question 5. A bus is moving towards a huge wall with a velocity of 5 m · s ^{-1}. The driver sounds a horn of frequency 200 Hz. The beat frequency heard by the passenger will be**

- 4
- 6
- 8
- 2

**Answer:** 2. 6

**Question 6. A motor car sounding a horn is approaching a large reflector. If the frequency of the horn is 1000 Hz, the frequency of the echo to the driver will be**

- 1000 Hz
- Less than 1000 Hz
- Greater than 1000 Hz
- None of these

**Answer:** 3. Greater than 1000 Hz

**Question 7. An observer standing on a railway crossing receives frequencies of 2.2 kHz and 1.8 kHz when the train approaches and recedes from the observer. The speed of the sound in air is 300 m · s ^{-1}. The velocity of the train (in m · s^{-1})**

- 60
- 30
- 90
- 70

**Answer:** 2. 30

**Question 8. A whistle producing sound waves of frequency 9500 Hz and above is approaching a stationary person with speed v m · s ^{-1}. The velocity of sound in air is 300 m · s^{-1}. If the person can hear frequencies upto a maximum of 10000 Hz, the maximum value of v up to which he can hear the whistle is**

- 15√2 m · s
^{-1} - 15/√2 m · s
^{-1} - 15 m · s
^{-1} - 30 m · s
^{-1}

**Answer:** 3. 15 m-s^{-1}

**Question 9. What do you understand by the red shift of a star or of a galaxy of stars?**

- They are gradually receding from the earth
- They are gradually approaching the earth
- They are stationary
- None of the above

**Answer:** 1. They are gradually receding from the earth

**Question 10. When a source of light and an observer approach each other, the apparent change of the wavelength of light is called**

- Redshift
- Violet shift
- Blueshift
- Black shift

**Answer:** 3. Blueshift

**In this type of questions, more than one options are correct.**

**Question 11. State in which of the following cases, an observer will not see any Doppler effect?**

- Both the source and observer remain stationary but a wind blows.
- The observer remains stationary but the source moves in the same direction and with the same speed as the wind.
- The source remains stationary but the observer and the wind have the same speed away from the source.
- The source and the observer move directly against the wind but both with the same speed.

**Answer:**

1. Both the source and observer remain stationary but a wind blows.

4. The source and the observer move directly against the wind but both with the same speed.

**Question 12. Consider a source of sound S and an observer P. The sound source is of frequency n _{0}. The frequency observed by P is found to be n_{1} if P approaches S at speed v and S is stationary, n_{2} if S approaches P at speed v and P is stationary and n_{3 }if each of P and S has speed \(\frac{v}{2}\) towards one another. Which of the following conclusions is correct?**

- n
_{1}= n_{2}= n_{3} - n
_{1}< n_{2} - n
_{3 }> n_{0} - n
_{3}lies between n_{1}and n_{2}

**Answer:**

2. n_{1}< n_{2}

3. n_{3}>n_{0}

4. n_{3} lies between n_{1} and n_{2}

**Question 13. An observer A is moving directly towards a stationary sound source while another observer B is moving away from the source with the same velocity. Which of the following conclusions are correct?**

- The average of frequencies recorded by A and B is equal to the natural frequency of the source.
- The wavelength of waves received by A is less than that of waves received by B.
- The wavelength of waves received by two observers will be the same.
- Both observers will observe the wave travelling at same speed.

**Answer:**

1. The average of frequencies recorded by A and B is equal to natural frequency of the source.

3. The wavelength of waves received by two observers will be the same.

**Question 14. Two cars, each moving with speed u on the same horizontal straight road, are approaching each other. Wind blows along the road with velocity w. One of these cars blows a whistle of frequency f _{1}. An observer in the other car hears the frequency of the whistle to be f_{2}. The speed of sound in still air is v. Correct statement(s) is are:**

- If the wind blows from the observer to the source, f
_{2}> f_{1} - If the wind blows from the source to observer, f
_{2}>f_{1} - If the wind blows from the observer to the source, f
_{2}<f_{1} - If the wind blows from the source to the observer, f
_{2}<f_{1}

**Answer:**

- If the wind blows from the observer to the source, f
_{2}> f_{1} - If the wind blows from the source to the observer, f
_{2}>f_{1} - If the wind blows from the observer to the source, f
_{2}<f_{1}