We hear an echo due to

Correct — B, Reflection of sound waves. An echo is the repetition of a sound heard after the original, produced when sound waves bounce back (reflect) from a distant hard surface such as a cliff or wall. To be heard as a separate echo the reflected sound must return after at least about 0.1 second, which needs the reflecting surface to be roughly 17 metres or more away (sound travels about 344 m/s in air).

Why the others are wrong

• (a)
  Refraction of sound waves — Refraction is the bending of sound as it passes between layers of different temperature or density — it does not send the sound back to produce an echo.
• (c)
  Diffraction of sound waves — Diffraction is the spreading of sound around obstacles or through openings; it explains why we hear around corners, not the return of an echo.
• (d)
  Resonance due to sound waves — Resonance is the large-amplitude vibration of a body driven at its natural frequency; it amplifies sound but does not create the delayed repeat that defines an echo.

Sound waves obey the same reflection rules as other waves. When they strike a large, hard surface they bounce back; if the reflected wave reaches the ear at least about 0.1 s after the direct sound, the brain perceives it as a separate echo. Multiple reflections merging together instead produce reverberation. The same principle underlies SONAR and echocardiography.

Key facts

• An echo is heard when sound reflects off a distant surface and returns after roughly 0.1 s or more.
• The minimum distance to hear a distinct echo in air is about 17 m (speed of sound ≈ 344 m/s at 22 °C).
• Many reflections merging together give reverberation rather than a clear echo.
• Echo (reflection of sound) is the basis of SONAR for measuring ocean depth and detecting objects.

Common traps

• Confusing reverberation (many overlapping reflections) with a distinct echo.
• Picking resonance or diffraction because they are also sound phenomena.

• practice — not a real PYQ

  The technique that uses reflection of sound (ultrasound) to measure the depth of the sea is

  • (a)RADAR
  • (b)SONAR
  • (c)LASER
  • (d)MRI
  Answer(b) SONAR — it times echoes of ultrasound reflected from the seabed to find the depth.
• practice — not a real PYQ

  The persistence of sound due to repeated reflections in a large hall is called

  • (a)Echo
  • (b)Reverberation
  • (c)Resonance
  • (d)Refraction
  Answer(b) Reverberation — overlapping reflections that prolong the sound rather than give a single clear echo.