Echo is produced by reflection of sound waves from a denser medium. What is the phase change between sound wave and echo?
Correct answer: B. 180 degrees
- A. 0 degrees
- B. 180 degrees
- C. 270 degrees
- D. 90 degrees
Explanation
When a sound wave is reflected from a denser medium, the reflected wave undergoes a phase change of 180 degrees or pi radians. This is because the denser medium causes the wave to undergo a reflection with a fixed boundary condition, and at the fixed boundary, the displacement of the particles of the medium is inverted while the pressure remains constant. This results in a phase shift of 180 degrees. So, when the reflected wave superimposes with the incident wave, they are out of phase by 180 degrees, leading to the formation of an echo.
This question appeared on the NUMS 2019 (cancelled sitting) paper, which you can sit online with every answer explained.
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