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From Echoes to Imaging: The Brilliant World of Ultrasound

Ultrasound refers to sound waves that have frequencies greater than 20 kHz, which is the upper limit of human hearing. Although these waves are inaudible to humans, they possess remarkable physical properties that make them extremely useful in science, medicine, and industry. Like all sound waves, ultrasound waves are mechanical longitudinal waves and require a material medium to propagate. They travel through media by producing successive compressions and rarefactions of particles, transferring energy and momentum without transferring matter.

From a physics perspective, ultrasound obeys the same fundamental wave equations as audible sound, but its very high frequency and short wavelength give it special characteristics. Because of the short wavelength, ultrasound can be sharply focused and reflected efficiently from small objects or boundaries. This makes ultrasound particularly suitable for imaging, detection, and measurement applications. The velocity of ultrasound depends on the elasticity and density of the medium, which is why its speed varies in solids, liquids, and gases.

One of the most important methods of producing ultrasound is the piezoelectric effect, where certain crystals (such as quartz) vibrate at ultrasonic frequencies when subjected to an alternating electric field. Another method involves magnetostriction, used mainly in industrial applications. These production techniques are frequently tested in competitive exams through conceptual MCQs.

Ultrasound plays a crucial role in medical diagnostics, most notably in ultrasonography, where it is used to visualize internal organs, monitor fetal development, and detect abnormalities without harmful radiation. Beyond medicine, ultrasound is widely used in SONAR to measure underwater distances, in non-destructive testing to detect flaws in metals, and in industrial cleaning and welding. Its applications clearly demonstrate how fundamental wave physics translates into powerful real-world technology.

Table of Contents

30 MCQs on Ultrasound with Answers:

1. The velocity of sound waves in a medium does not depend on:

A. Temperature
B. Pressure
C. Humidity
D. Direction of air
Answer: B

2. A tuning fork arrangement produces 4 beats/s with a fork of frequency 288 Hz. After applying wax, beats reduce to 2 beats/s. The frequency of the unknown fork is:

A. 286 Hz
B. 292 Hz
C. 294 Hz
D. 288 Hz
Answer: B

3. If two frequencies are (ν − 1) and (ν + 1), the beat frequency is:

A. 2
B. 1
C. 3
D. 4
Answer: A

4. The frequency of ultrasound waves is:

A. Less than 20 Hz
B. Between 20 Hz and 2 kHz
C. Between 2 kHz and 20 kHz
D. Greater than 20 kHz
Answer: D

5. When a sound wave travels from water to air, it:

A. Bends towards the normal
B. Bends away from the normal
C. May bend in any direction
D. Data insufficient
Answer: A

6. A sound wave of frequency 300 Hz has a maximum displacement of 0.1 cm. The maximum velocity of the particle is:

A. 60 cm/s
B. 30 cm/s
C. 6 cm/s
D. 30 cm/s
Answer: A

7. Which frequency is audible to humans?

A. 100 kHz
B. 40 kHz
C. 2 kHz
D. 30 kHz
Answer: C

8. A wave of frequency 500 Hz travels at 360 m/s. Distance between two nearest points 60° out of phase is:

A. 12 cm
B. 18 cm
C. 30 cm
D. 6 cm
Answer: A

9. Two waves each of loudness L superimpose to produce beats. Maximum loudness is:

A. 4L
B. L
C. 2L
D. 5L
Answer: A

10. Pressure variations during sound propagation are:

A. Isobaric
B. Isochoric
C. Adiabatic
D. Isothermal
Answer: D

11. The physical quantity that remains unchanged when sound enters another medium is:

A. Amplitude
B. Speed
C. Wavelength
D. Frequency
Answer: D

12. Speed of sound in a gas is proportional to:

A. (ρ/p)²
B. (ρ/p)³ᐟ²
C. ρ/p
D. p/ρ
Answer: D

13. The beat frequency of two tuning forks of frequencies ν₁ and ν₂ is:

A. ν₁ + ν₂
B. ν₁ × ν₂
C. ν₂ / ν₁
D. |ν₁ − ν₂|
Answer: D

14. Motion of an air molecule due to sound from a distant wolf is:

A. Back and forth oscillation
B. Circular motion
C. Vertical oscillation
D. Motion with speed of sound
Answer: A

15. Two waves of wavelengths 100 cm and 101 cm travel at 303 m/s. Beat frequency is:

A. 4 Hz
B. 1 Hz
C. 3 Hz
D. 2 Hz
Answer: C

16. Ultrasonic waves are produced by:

A. Piezoelectric effect
B. Peltier effect
C. Doppler effect
D. Coulomb’s law
Answer: A

17. Which statement is correct?

A. Both light and sound are transverse
B. Sound is longitudinal, light is transverse
C. Both are longitudinal
D. Both travel in vacuum
Answer: B

18. Sound waves transfer:

A. Only energy
B. Energy
C. Momentum
D. Energy and momentum
Answer: D

19. A tuning fork completes 25 vibrations while sound travels 16.5 m. Frequency = 500 Hz. Velocity of sound is:

A. 350 m/s
B. 330 m/s
C. 300 m/s
D. 450 m/s
Answer: B

20. Zero level of sound intensity corresponds to:

A. 10⁻¹⁰ W/m²
B. 10⁻¹² W/m²
C. 10⁻¹⁴ W/m²
D. 10⁻¹⁶ W/m²
Answer: B

21. Velocity of sound is greatest in:

A. Water
B. Air
C. Steel
D. Vacuum
Answer: D

22. End correction of an open pipe is 0.8 cm. Inner radius is:

A. 1/3 cm
B. 2/3 cm
C. 3/2 cm
D. 0.2 cm
Answer: B

23. When a stationary wave has maximum kinetic energy, the string appears:

A. Sinusoidal with amplitude 3A
B. Sinusoidal with amplitude 2A
C. Sinusoidal with amplitude A
D. Straight line
Answer: D

24. Statements:

(a) Velocity of sound is higher in less compressible media
(b) Velocity of sound is higher in solids than gases
Correct option:
A. Only (a)
B. Only (b)
C. Both (a) and (b)
D. Neither
Answer: C

25. Sound travels in an inert gas at constant temperature while pressure increases. Velocity will:

A. Increase
B. Decrease
C. Remain constant
D. Depend on pressure
Answer: C

26. A siren disc with 60 holes rotates at 360 rpm. Frequency of sound is:

A. 2 Hz
B. 4 Hz
C. 8 Hz
D. 16 Hz
Answer: B

27. In Melde’s experiment (transverse mode), frequency ratio (fork : string) is:

A. 2 : 1
B. 4 : 1
C. 1 : 1
D. 1 : 2
Answer: C

28. Loudness and pitch depend on:

A. Intensity & velocity
B. Frequency & velocity
C. Intensity & frequency
D. Frequency & harmonics
Answer: C

29. Sound waves in air are longitudinal because:

A. Nature of sound
B. Air has no rigidity
C. Air is a gas mixture
D. Low density
Answer: B

30. Shortest length of a closed pipe resonating at frequency n (speed = v) is:

A. 2nv
B. 4v/n
C. n²v
D. 2v/n
Answer: B

ultrasound

Conclusion: Importance of Ultrasound in Physics and Applications

Ultrasound is a perfect example of how a simple physical concept—sound waves—can evolve into a highly impactful scientific and technological tool. Despite being inaudible to humans, ultrasonic waves play a vital role in modern life due to their high penetrating power, directionality, and sensitivity to material boundaries. From a theoretical standpoint, ultrasound reinforces key ideas related to wave motion, frequency, wavelength, velocity, reflection, refraction, and interference, all of which are central to physics education.

In examinations, questions on ultrasound often test conceptual clarity rather than lengthy calculations. Aspirants are expected to understand why ultrasound waves are safe for medical imaging, why they travel faster in solids than in gases, and why their frequency remains unchanged when passing from one medium to another. Topics such as piezoelectric production, medical imaging principles, SONAR applications, and industrial uses are especially important for MCQs and assertion-reason questions.

Beyond academics, ultrasound demonstrates the practical relevance of physics in solving real-world problems. Its non-invasive nature has revolutionized medical diagnostics, while its precision has improved quality control in industries and navigation in marine environments. As technology advances, the applications of ultrasound continue to expand, making it a topic of lasting importance.

In summary, ultrasound bridges the gap between fundamental wave physics and advanced technological applications. A strong conceptual understanding of ultrasound not only helps aspirants excel in competitive exams but also deepens their appreciation of how physics principles shape modern science and engineering.

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