CBSE Class 10 Physics Waves and Sound MCQs Set 20

Answer: (d) f = 1/T

Explanation:
1. Frequency is the number of oscillations per second, and the time period is the time for one oscillation.
2. So f = 1/T.

Teacher's Note:
1. Also v = f\( \lambda \).

Answer: (c) an oscillation to pass through a point

Explanation:
1. One full wave (one compression plus one rarefaction) passing a point takes one time period.

Teacher's Note:
1. The time period is the time of one oscillation.

Answer: (c) the amplitude of vibration is very large

Explanation:
1. At resonance the driving frequency equals the natural frequency.
2. Energy is absorbed most efficiently and the amplitude becomes very large.

Teacher's Note:
1. Resonance is a special case of forced oscillation.

Answer: (c) \( \lambda \)/2

Explanation:
1. A crest and the next trough are separated by half of a complete wave.
2. The distance is \( \lambda \)/2.

Teacher's Note:
1. The distance between two consecutive crests is \( \lambda \).

Answer: (c) 1.12 s

Explanation:
1. Mean to extreme is one quarter of an oscillation, so T/4 = 0.28 s.
2. T = 4 \( \times \) 0.28 = 1.12 s.

Teacher's Note:
1. One full oscillation covers four quarter-swings.

Answer: (c) of one wavelength in air

Explanation:
1. Distance = v \( \times \) T.
2. Since \( \lambda \) = vT, the sound travels one wavelength in one period.

Teacher's Note:
1. This is the physical meaning of wavelength.

Answer: (b) 20 mm

Explanation:
1. The highest audible frequency is 20,000 Hz.
2. \( \lambda \) = v/f = 400/20000 = 0.02 m = 20 mm.

Teacher's Note:
1. Smallest wavelength corresponds to the highest audible frequency.

Answer: (b) longitudinal waves

Explanation:
1. Air can support only compressions and rarefactions.
2. Sound in air is a longitudinal wave.

Teacher's Note:
1. Gases cannot sustain transverse mechanical waves.

Answer: (c) oscillatory motion of particles in the medium

Explanation:
1. Sound is produced by vibrations.
2. Each particle of the medium oscillates to and fro about its mean position.

Teacher's Note:
1. Particles do not travel along with the wave.

Answer: (c) ultrasonic waves

Explanation:
1. Ultrasonography sends ultrasonic waves into the body and forms an image from the echoes.

Teacher's Note:
1. These waves are safe compared with X-rays.

Answer: (d) all of the above

Explanation:
1. Longitudinal waves need only elasticity of volume, which every state of matter has.
2. They travel in solids, liquids and gases.

Teacher's Note:
1. Transverse mechanical waves travel only in solids and on liquid surfaces.

Answer: (c) more than 1/10 s

Explanation:
1. The ear retains a sound impression for about 0.1 s.
2. The reflected sound must arrive after more than 1/10 s to be heard separately.

Teacher's Note:
1. At 340 m/s this needs an obstacle at least 17 m away.

Answer: (a) minimum

Explanation:
1. Minimum density means a rarefaction.
2. Pressure is also minimum in a rarefaction.

Teacher's Note:
1. Density and pressure go up and down together in a sound wave.

Answer: (b) vocal cords

Explanation:
1. The cochlea, hammer (malleus) and auditory nerve are all in the ear.
2. Vocal cords are in the larynx and produce sound.

Teacher's Note:
1. Vocal cords are used for speaking, not hearing.

Answer: (b) 100 Hz

Explanation:
1. 20 crests and 20 troughs make 20 complete waves.
2. f = 20/0.2 = 100 Hz.

Teacher's Note:
1. One crest plus one trough is one complete oscillation.

Answer: (b) compressions and rarefactions in the medium

Explanation:
1. In a longitudinal wave the particles vibrate along the direction of travel.
2. This produces alternate regions of high density (compressions) and low density (rarefactions).

Teacher's Note:
1. Crests and troughs belong to transverse waves.

Answer: (b) infrasound

Explanation:
1. Earthquakes produce low-frequency waves below 20 Hz.
2. Such waves are infrasound. Some animals sense them.

Teacher's Note:
1. Infrasound has a frequency below 20 Hz.

Answer: (d) the sound is louder but the pitch is not different

Explanation:
1. Striking harder gives a larger amplitude, so the loudness increases.
2. The string vibrates with the same frequency, so the pitch is unchanged.

Teacher's Note:
1. Loudness depends on amplitude; pitch depends on frequency.

Answer: (d) 819

Explanation:
1. Speed is proportional to the square root of the absolute temperature.
2. For double speed the absolute temperature must be 4 times: 4 \( \times \) 273 = 1092 K.
3. 1092 - 273 = 819 \( ^\circ \)C.

Teacher's Note:
1. Always convert to kelvin when using v proportional to \( \sqrt{T} \).

Answer: (a) of a single frequency

Explanation:
1. A note is a sound of a single definite frequency.
2. A mixture of several frequencies is called a tone, and unpleasant sound is noise.

Teacher's Note:
1. Notes are used in music; a tuning fork gives a pure note.

Answer: (a) 781 m

Explanation:
1. Distance = v \( \times \) t/2 = 1531 \( \times \) 1.02/2.
2. = 780.8 m, about 781 m.

Teacher's Note:
1. Only half of the travelled path is the distance of the cliff.

Answer: (a) ultrasonic waves

Explanation:
1. The audible range is 20 Hz to 20,000 Hz.
2. Frequencies above this range are ultrasonic.

Teacher's Note:
1. Supersonic refers to a speed greater than that of sound, not to frequency.

Answer: (d) both (a) and (b)

Explanation:
1. Wavelength is the distance between two successive points in the same phase.
2. Two consecutive compressions or two consecutive rarefactions are such points.

Teacher's Note:
1. A compression and the next rarefaction are only half a wavelength apart.

Answer: (b) sonar

Explanation:
1. Sonar uses reflected ultrasonic pulses to find the position of underwater objects.

Teacher's Note:
1. Ultrasonography is used for medical imaging.