CBSE Class 9 Physics Waves and Sound MCQs Set 15

Download CBSE MCQs for Class 9 Physics: Waves and Sound

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Chapter-wise Objective Questions: Waves and Sound

Access the complete set of multiple-choice questions for Waves and Sound below. This focused format allows students to isolate specific topics for thorough review and uninterrupted practice alongside official CBSE textbooks.

Multiple Choice Questions

Question 1: An observer standing on the sea-coast observes 54 waves reaching the coast per minute. If the wavelength of the wave is 10 m, the velocity of the wave will be
(a) 4.5 m/s
(b) 9 m/s
(c) 18 m/s
(d) 27 m/s
Show Answer & Explanation

Answer: (b) 9 m/s

Explanation:
1. Frequency = 54/60 = 0.9 Hz.
2. v = fλ = 0.9 × 10 = 9 m/s.

Teacher's Note:
1. Convert waves per minute into waves per second.

Question 2: Non-mechanical waves can travel
(a) in vacuum as well as in a medium
(b) in vacuum but not in a medium
(c) in a medium but not in vacuum
(d) neither in a medium nor in vacuum
Show Answer & Explanation

Answer: (a) in vacuum as well as in a medium

Explanation:
1. Non-mechanical waves such as light and radio waves need no medium.
2. They can also pass through a medium, with a changed speed.

Teacher's Note:
1. Light travels through glass, water and vacuum.

Question 3: It is possible to distinguish between transverse and longitudinal waves by studying the property of
(a) interference
(b) diffraction
(c) reflection
(d) polarization
Show Answer & Explanation

Answer: (d) polarization

Explanation:
1. Only transverse waves can be polarised.
2. Interference, diffraction and reflection occur for both kinds of waves.

Teacher's Note:
1. Polarisation needs vibrations perpendicular to the direction of travel.

Question 4: The period of a pendulum whose length is 1 m is (take g = 9.8 m/s²)
(a) 2 s
(b) 1 s
(c) 6 s
(d) 10 s
Show Answer & Explanation

Answer: (a) 2 s

Explanation:
1. T = 2π√(1/9.8) = 2π × 0.3194 = 2.007 s.
2. So T is about 2 s, the second's pendulum.

Teacher's Note:
1. A pendulum about 1 m long is a second's pendulum.

Question 5: If the density of air at a point through which a sound wave is passing is maximum at an instant, the pressure at that point will be
(a) minimum
(b) the same as the density of air
(c) equal to the atmospheric pressure
(d) maximum
Show Answer & Explanation

Answer: (d) maximum

Explanation:
1. Maximum density occurs in a compression.
2. A compression is a region of maximum pressure.

Teacher's Note:
1. Compressions are high-pressure, high-density regions.

Question 6: The velocity of sound in air is not affected by a change in
(a) moisture content of the air
(b) the temperature of the air
(c) the atmospheric pressure
(d) the composition of air
Show Answer & Explanation

Answer: (c) the atmospheric pressure

Explanation:
1. Speed of sound depends on temperature, humidity and the composition of the gas.
2. At constant temperature a change in pressure changes the density in the same proportion, so the speed stays the same.

Teacher's Note:
1. v = √(γP/ρ), and P/ρ is constant at constant temperature.

Question 7: A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m/s.
(a) 16.2 m, 1.62 cm
(b) 16.2 m, 1.72 cm
(c) 16.7 m, 1.67 cm
(d) 17.2 m, 1.72 cm
Show Answer & Explanation

Answer: (d) 17.2 m, 1.72 cm

Explanation:
1. λ = v/f.
2. At 20 Hz: 344/20 = 17.2 m.
3. At 20,000 Hz: 344/20000 = 0.0172 m = 1.72 cm.

Teacher's Note:
1. The lower the frequency, the longer the wavelength.

Question 8: Consider the following statements:
A. Both sound and light waves are longitudinal.
B. Sound wave is longitudinal and light wave is transverse.
C. Sound wave is transverse and light wave is longitudinal.
D. Both sound and light waves are transverse.
The correct statement is
(a) A
(b) B
(c) C
(d) D
Show Answer & Explanation

Answer: (b) B

Explanation:
1. Sound in air is a longitudinal wave of compressions and rarefactions.
2. Light is an electromagnetic wave, which is transverse.

Teacher's Note:
1. Only B states both facts correctly.

Question 9: Two waves travelling in a medium in the x-direction are represented by y1 = A sin(αt − βx) and y2 = A cos(βx + αt − π/4), where y1 and y2 are the displacements of the particles of the medium, t is time, and α and β are constants. The two waves have different
(a) speeds
(b) directions of propagation
(c) wavelengths
(d) frequencies
Show Answer & Explanation

Answer: (b) directions of propagation

Explanation:
1. In y1 the combination (αt − βx) means the wave moves in the +x direction.
2. In y2 the combination (αt + βx) means the wave moves in the −x direction.
3. Both have the same frequency (α), the same wavenumber (β), and so the same wavelength and speed.

Teacher's Note:
1. The relative sign of t and x gives the direction.

Question 10: If a wave completes 20 vibrations in 2.5 s, then its frequency is
(a) 20 Hz
(b) 8 Hz
(c) 200 Hz
(d) 50 Hz
Show Answer & Explanation

Answer: (b) 8 Hz

Explanation:
1. Frequency = number of vibrations / time = 20/2.5 = 8 Hz.

Teacher's Note:
1. Frequency is vibrations per second.

Question 11: A body is vibrating 6,000 times in one minute. If the velocity of sound in air is 360 m/s, the wavelength of the wave produced is
(a) 2.6 m
(b) 3.6 m
(c) 2.5 m
(d) 5.5 m
Show Answer & Explanation

Answer: (b) 3.6 m

Explanation:
1. Frequency = 6000/60 = 100 Hz.
2. λ = v/f = 360/100 = 3.6 m.

Teacher's Note:
1. Convert vibrations per minute to hertz.

Question 12: A wave pulse on a string moves a distance of 8 m in 0.04 s. What would be the wavelength of a wave on the same string if its frequency is 250 Hz?
(a) 0.4 m
(b) 0.5 m
(c) 0.8 m
(d) 0.10 m
Show Answer & Explanation

Answer: (c) 0.8 m

Explanation:
1. Speed v = 8/0.04 = 200 m/s.
2. λ = v/f = 200/250 = 0.8 m.

Teacher's Note:
1. The speed on the string is fixed by the string, not by the frequency.

Question 13: If the string of a pendulum were cut when the bob is at its central position, the bob would fall to the earth due to the absence of the
(a) force of buoyancy
(b) force of deformation
(c) force exerted by the string in the downward direction
(d) force exerted by the string in the upward direction
Show Answer & Explanation

Answer: (d) force exerted by the string in the upward direction

Explanation:
1. At the mean position the string pulls the bob upward, which together with gravity gives the centripetal force.
2. On cutting the string the upward pull vanishes, so gravity makes the bob fall.

Teacher's Note:
1. The tension is directed upward at the lowest point.

Question 14: Transverse waves can propagate
(a) both in a gas and a metal
(b) in a gas but not in a metal
(c) not in a gas but in a metal
(d) neither in a gas nor in a metal
Show Answer & Explanation

Answer: (c) not in a gas but in a metal

Explanation:
1. Transverse waves need a medium with rigidity (shear elasticity).
2. Solids such as metals have it, but gases do not.

Teacher's Note:
1. Gases and liquids cannot support transverse waves in the bulk.

Question 15: When the length of a pendulum is doubled and the mass of its bob is halved, its time period
(a) becomes double
(b) becomes half
(c) becomes √2 times
(d) remains the same
Show Answer & Explanation

Answer: (c) becomes √2 times

Explanation:
1. The mass has no effect on the period.
2. T ∝ √L, so doubling L multiplies T by √2.

Teacher's Note:
1. Ignore the mass.

Question 16: Two particles are out of phase. What is the minimum path difference between them in terms of wavelength?
(a) λ
(b) 2λ
(c) λ/2
(d) 4λ
Show Answer & Explanation

Answer: (c) λ/2

Explanation:
1. Out of phase means a phase difference of π.
2. A path difference of λ corresponds to 2π, so π corresponds to λ/2.

Teacher's Note:
1. Phase difference = 2π × path difference / λ.

Question 17: The height of the crests of a wave is called its
(a) amplitude
(b) displacement
(c) frequency
(d) angular frequency
Show Answer & Explanation

Answer: (a) amplitude

Explanation:
1. The maximum displacement from the mean position, that is the height of a crest, is the amplitude.

Teacher's Note:
1. Amplitude is measured from the mean line.

Question 18: Though the forces are balanced at the mean position, the bob crosses over to the other extreme position after being released because of
(a) inertia of the bob
(b) potential energy of the bob
(c) velocity of the bob
(d) none of these
Show Answer & Explanation

Answer: (a) inertia of the bob

Explanation:
1. At the mean position the net force is zero, but the bob has speed.
2. Inertia keeps it moving past the mean position.

Teacher's Note:
1. Inertia carries a body beyond the equilibrium point.

Question 19: If two waves of the form y = a sin(ωt − kx) and y = a cos(kx − ωt) are superposed, the resultant wave will have amplitude
(a) 0
(b) a
(c) √2 a
(d) 2a
Show Answer & Explanation

Answer: (c) √2 a

Explanation:
1. cos(kx − ωt) = cos(ωt − kx).
2. The sum is a[sin θ + cos θ] = a√2 sin(θ + 45°), where θ = ωt − kx.
3. The amplitude is √2 a.

Teacher's Note:
1. Combine sine and cosine of the same angle.

Question 20: The speed of light in water is 2.25 × 108 m/s. If the speed of light in vacuum is 3 × 108 m/s, the refractive index of water is
(a) 0.33
(b) 1.33
(c) 1.04
(d) 2.43
Show Answer & Explanation

Answer: (b) 1.33

Explanation:
1. Refractive index = speed in vacuum / speed in medium.
2. n = 3 × 108 / 2.25 × 108 = 1.33.

Teacher's Note:
1. Refractive index is always greater than 1.

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