CAPF AC 2025 Q106: Science & Technology – Physics
- (a) Resonance
- (b) Superposition of waves
- (c) Doppler effect
- (d) Standing wave
Answer: (c) Doppler effect
Explanation
Principle used in sonography
How to think: sonography sends ultrasound into the body and builds an image from the echoes.
Of the four options, the one used to measure the movement of blood and organs from the echoes is the Doppler effect.
| Principle | Use |
|---|---|
| Resonance | Tuning of instruments, MRI relies on a different resonance |
| Superposition of waves | Interference and beats |
| Doppler effect | Change of frequency of the echo from moving tissue and blood in Doppler sonography |
| Standing waves | Musical instruments and resonance tubes |
Note: the basic picture in sonography comes from the reflection of ultrasound (the echo). Reflection is not offered, so (c) is the best option.
Answer: (c) Doppler effect.
Why each option is right or wrong
- (a) Incorrect. Resonance is not the basis of ultrasound imaging.
- (b) Incorrect. Superposition of waves explains interference, not the imaging.
- (c) CORRECT. The Doppler effect is used in sonography to study moving blood and tissue.
- (d) Incorrect. Standing waves are not used to form the image.
Key point
Ultrasound imaging: pulses of 1-15 MHz, echoes from tissue boundaries. Doppler sonography: frequency shift of echoes gives the speed of blood flow. Ultrasound frequency is above 20 kHz.
Remember
Sonography: ultrasound echoes; Doppler effect measures motion.
Common mistakes
- Choosing resonance because MRI and instruments use it.
- Forgetting that the basic imaging uses reflection of sound waves.
- Confusing sonography (sound) with X-ray or MRI.
Exam tip
Educated guess: a scan of a moving heart or blood flow depends on frequency changes of the echo. That is the Doppler effect, the only option that links to motion.
Sources
- NCERT Class IX Science, Chapter 12 (Sound: applications of ultrasound, medical imaging)
- Halliday, Resnick and Walker, Fundamentals of Physics, chapter on the Doppler effect