In Stethoscope, the sound of the patient’s heartbeat reaches the doctor’s ears by -

Correct — option (2), Multiple reflection of sound.

NCERT's Class 9 Science chapter Sound (2021-22 edition) lists the stethoscope under uses of multiple reflection of sound: "In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound".

NCERT explains the idea with megaphones and horns: a tube reflects sound successively to guide most of it forward instead of letting it spread. The stethoscope uses the same effect to carry the heartbeat along its tubes to the doctor.

So the sound reaches the ears by multiple reflection of sound.

The idea to remember: stethoscope, megaphone and horn all guide sound by repeated reflection.

Why the others are wrong

• (1)
  Multiple diffraction of sound — Diffraction is sound bending around obstacles and through openings. NCERT's Class 12 Physics chapter Wave Optics gives the everyday case: we are hardly surprised to hear someone talking around a corner.

  That spreads sound into regions it would not reach in a straight line. A stethoscope does the opposite: it keeps the sound inside a tube and guides it to one listener by reflection.

• (3)
  Polarisation of sound — Sound in air cannot be polarised. NCERT's Class 9 chapter says sound waves are longitudinal waves.

  NCERT's Class 12 Wave Optics chapter adds: "Most interference and diffraction effects exist even for longitudinal waves like sound in air. But polarisation phenomena are special to transverse waves like light waves."

• (4)
  Multiple refraction of sound — Refraction is the bending of a wave as it passes from one medium into another. Sound does change speed between media: NCERT's table gives 346 m/s in air and 1531 m/s in sea water at 25 °C.

  But the stethoscope's action is not described that way. NCERT names multiple reflection, the same effect that guides sound through a megaphone's tube.

Sound bounces off a solid or a liquid much as a ball bounces off a wall. NCERT says reflected sound follows the same laws of reflection as light: the incident and reflected directions make equal angles with the normal, and all three lie in one plane.

When sound reflects again and again, the effect can be used. NCERT lists megaphones, horns, trumpets and shehanais; the stethoscope; curved ceilings of concert and conference halls; and soundboards placed behind a stage.

Key facts

• NCERT: in stethoscopes, the sound of the patient's heartbeat reaches the doctor's ears by multiple reflection of sound.
• NCERT: reflected sound follows the same laws of reflection as light.
• NCERT: megaphones, horns, trumpets, shehanais, curved hall ceilings and soundboards also use multiple reflection of sound.
• NCERT: taking sound's speed as 344 m/s, a distinct echo needs an obstacle at least 17.2 m away.
• NCERT Class 12: polarisation is special to transverse waves, while sound in air is a longitudinal wave.

Common traps

• A tube does not mean refraction: NCERT says the stethoscope carries sound by multiple reflection.
• Sound in air cannot be polarised: polarisation belongs to transverse waves, and sound in air is longitudinal.
• Diffraction spreads sound around corners; it does not channel sound along a tube to one listener.

UnlockIAS compared this question with questions from other RAS Prelims papers and found none similar enough to link.

• practice — not a real PYQ

  Which of the following phenomena is NOT shown by sound waves in air?

  • (a)Reflection
  • (b)Diffraction
  • (c)Interference
  • (d)Polarisation
  Answer(4) — NCERT says polarisation phenomena are special to transverse waves, and sound in air is longitudinal. NCERT says sound is reflected (option (1)) and that interference and diffraction effects exist even for sound in air (options (3) and (2)).
• practice — not a real PYQ

  Taking the speed of sound in air as 344 m/s and the persistence of hearing as 0.1 s, the minimum distance of a reflecting obstacle for a distinct echo is –

  • (a)34.4 m
  • (b)17.2 m
  • (c)3.44 m
  • (d)344 m
  Answer(2) — sound must travel to the obstacle and back in at least 0.1 s: 344 × 0.1 = 34.4 m for the round trip, and half of that is 17.2 m. Option (1) is the round-trip distance itself, option (3) is the round trip for 0.01 s, and option (4) the round trip for 1 s.