NEET Physics Wave Motion
Each of the following questions comprises two statements. The assertion is expressed by Statement 1 and the reason is expressed by Statement 2.
Each question has four options (1), (2), (3), and (4) as given below, out of which only ONE is correct.
- Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
- Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
- Statement 1 is true and Statement 2 is false.
- Statement 1 is false and Statement 2 is true.
- Both Statement 1 and Statement 2 are false.
Some questions are preceded, by a supporting paragraph in addition to the statements.
NEET Physics Sound Waves
Question 1. Statement 1 The flash of lightning is seen before the sound of thunder is heard.
Statement 2 The speed of sound is greater than the speed of light.
Answer: 3. Statement 1 is true and Statement 2 is false.
The speed of light (= 3 x 108 m s-1) is much greater than the speed of sound (= 330 m s-1). So, the flash is observed much earlier.
Question 2. Statement 1 Sound waves cannot propagate through a vacuum but light waves can.
Statement 2 Sound waves cannot be polarized but light waves can be.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
Light waves, which are electromagnetic in nature, can travel through a vacuum. But sound waves are mechanical elastic waves, which need an elastic medium that can undergo periodic compression and expansion. Polarization occurs only in transverse waves (like light) and not in longitudinal waves (like sound waves).
Question 3. Statement 1 To hear distinct beats, the difference in frequencies of two sources should be less than ten.
Statement 2 The more the number of beats per second, the more difficult it is to hear them.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The sensation of hearing of any sound persists in our brain for 0.1 s, so for distinct hearing of beats the difference in frequencies (= number of beats per second) must be less than 10.
Question 4. Statement 1 Transverse sound waves do not occur in gases.
Statement 2 Gases cannot sustain shearing strain.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
Sound is a longitudinal wave motion. Gases can undergo only volume strain and not shear strain.
Question 5. Statement 1 The fundamental frequency of an organpipe increases as the temperature is increased.
Statement 2 This is because as the temperature increases, the velocity of sound increases more rapidly than the length of the pipe.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The fundamental frequency (η) of an open organ pipe, \(\eta=\frac{v}{2 l}\) increases with temperature as the speed of sound v increases faster than length l.
Question 6. Statement 1 When two vibrating tuning forks having frequencies 256 Hz and 512 Hz are held near each other, beats cannot be heard.
Statement 2 The principle of superposition is valid only if the frequencies of the oscillators are nearly equal.
Answer: 3. Statement 1 is true and Statement 2 is false.
Beats can be heard only if the beat frequency is less than 10 s-1. In this case, Δn = 256 s-1. The principle of superposition holds for any difference in frequency.
Question 7. Statement 1 In a stationary wave system, the displacement nodes are pressure antinodes and displacement antinodes are pressure nodes.
Statement 2 When a closed organ pipe is set into vibration, the pressure of the gas at the closed end remains constant.
Answer: 3. Statement 1 is true and Statement 2 is false.
When stationary waves are set up in a closed organ pipe, displacement nodes are produced at the closed end where pressure antinodes are formed. At pressure antinodes, pressure/density undergoes the maximum variation.
Question 8. Statement 1 When two identical strings stretched to slightly different tensions vibrate together, the loudness of sound heard changes periodically.
Statement 2 Interference can occur in all wave motions under suitable conditions.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
Interference is a wave phenomenon exhibited by both longitudinal (sound) and transverse (light) waves.
The vibration of strings with slightly different frequencies produces waves that produce beats due to interference in time. This causes the periodic maximum and minimum of loudness.
Question 9. Statement 1 When two sounds of slightly different frequencies are heard together, periodic variations in intensity (called beats) are observed. A similar phenomenon is not observed when two lights of slightly different wavelengths reach a point and superpose.
Statement 2 Sound waves are longitudinal in nature, while light waves are transverse.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
The superposition of sound waves from two independent sources having a small frequency difference produces beats, which is a rhythmic variation in intensity with time. A similar phenomenon with light is not normally observed because the light we have around us comes from incoherent sources. For such a production in light, we need a laser source. The laser beam can be split into two components. The frequency of one beam can be changed appropriately. Their superposition will lead to the desired result.
Question 10. Statement 1 An open organ pipe can be used as a musical instrument but not a closed organ pipe.
Statement 2 The fundamental frequency of an open organ pipe is twice the fundamental frequency of a closed organ pipe of the same length.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
In an open organ pipe, all the harmonics of the fundamental are formed. This richness in overtone is the cause of the melodious note. In a closed organ pipe, only odd harmonics are present. The absence of (even) harmonics makes the note nonmelodious. Fundamental frequencies in closed and open pipes are \(f_{\mathrm{c}}=\frac{v}{4 l} \text { and } f_{\mathrm{o}}=\frac{v}{2 l}\), where v =speed of sound.
Question 11. Statement 1 The Doppler effect occurs in all wave motions.
Statement 2 The Doppler effect can be explained by the principle of superposition of waves.
Answer: 3. Statement 1 is true and Statement 2 is false.
The Doppler effect corresponds to an apparent change in the frequency of both sound (longitudinal) and light (transverse) wave motions. This effect has nothing to do with the superposition but it explains how fast the crest or trough reaches the observer due to the relative motion between the source and the observer.