Sound waves

Sound waves is a Science & Technology concept from the MPSC syllabus, tested in 6 past MPSC questions across 5 exam years between 2014 and 2022. Below are those real questions with their correct answers and explanations, plus the concepts worth being comfortable with first.

Real questions

Years tested

5 · 2014–2022

In context

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Asked in

2022 2021 2020 2017 2014

What you need to know first

Audible frequency rangeMechanical wavesBoundary conditions for wavesFormula v = fλFormula Distance = speed × timev = f * lambda formulaHuman hearing frequency rangeNature of longitudinal waves

Real questions that test this

श्रव्यातीत तरंग असे तरंग आहेत की ते Ultrasonic waves are those waves which

a)(1) मानव ऐकू शकत नाही / human beings cannot hear✓ Correct

b)(2) मानव ऐकू शकतो / human beings can hear

c)(3) त्यांची वारंवारता कमी आहे / have less frequency

d)(4) वरीलपैकी एकही नाही / None of the above

Ultrasonic waves have frequencies above the human hearing range (greater than 20 kHz), hence they cannot be heard by human beings.

प्रकाश तरंग मुख्यत: ____________ स्वरूपात असतात. Light is generally in the form of :

1)चुंबकीय तरंग / Magnetic Waves

2)विद्युत तरंग / Electrical Waves

3)विद्युत चुंबकीय तरंग / Electromagnetic Waves✓ Correct

4)वरीलपैकी एकही नाही / None of the above

Light travels in the form of electromagnetic waves, specifically transverse waves.

जेव्हा एखाद्या ध्वनी तरंगाचे संपीड़न दृढ भंतीवर आदळेल तेव्हा त्याचे प्रतिबिंबित रूप हे : When a compression of sound waves is incident on rigid wall, it is reflected as :

1)180 अंशानी (π) अवस्था संकुचीत होईल. / Compression with a phase change of π (by 180°)

2)अवस्था बदल होणार नाही. / Compression with no phase change✓ Correct

3)90 अंशानी (π/2) अवस्था विरलन होईल. / Rarefaction with a phase change of π/2 (by 90°)

4)विरलन होईल पण अवस्था बदलणार नाही. / Rarefaction with no phase change

At a rigid boundary, particle displacement has a node and pressure has an antinode -- the opposite of what happens at a free/open boundary. This means a pressure compression reflects back as a compression (not a rarefaction), with no phase change in the pressure wave. (The displacement wave does invert by 180 degrees, but the question is about the compression/pressure wave itself.)

एका ध्वनी तरंगाची वारंवारता 1000 हर्ट्झ असून तरंगलांबी 0.25 मीटर आहे. जर तो एका विशिष्ट माध्यमातून 5 सेकंद प्रवास करत असेल, तर कापलेले अंतर काढा. Calculate the distance travelled by a sound wave having frequency 1000 Hz and wavelength 0.25 m, if it travels for 5 seconds in a certain medium.

1)50 km

2)1250 m✓ Correct

3)800 m

4)80 km

Speed of sound v = frequency * wavelength = 1000 Hz * 0.25 m = 250 m/s. Distance = speed * time = 250 m/s * 5 s = 1250 meters.

विशेष वारंवारता असणारा एका ध्वनी तरंगाचा वेग 336 मी./से. असून तरंग लांबी 3 सेमी आहे. तर वारंवारता काढा. ती श्रवणीय असेल का ? A sound wave having certain frequency travels with a speed of 336 m/s and wavelength 3 cm. What will its frequency be ? Will it be audible ?

1)100800 Hz, नाही / 100800 Hz, No

2)100800 Hz, होय / 100800 Hz, Yes

3)11200 Hz, नाही / 11200 Hz, No

4)11200 Hz, होय / 11200 Hz, Yes✓ Correct

Using formula v = f * λ, frequency f = v / λ = 336 / 0.03 = 11200 Hz = 11.2 kHz. Since human audible range is 20 Hz to 20,000 Hz, 11200 Hz is audible.

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