Class 11 Physics, Chapter 14 (NCERT). 15 questions by concept, 2 with pictures. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
A source sends out visible light and sound that travels through air. Which type of signal can cross an evacuated gap to a receiver?
(a) Electromagnetic visible light.
Mechanical waves need an elastic material medium. Electromagnetic radiation can propagate through vacuum.
A transparent bell jar contains a vibrating tuning fork beside a lamp. As the air is removed, why does the light stay visible while the sound, which travels through air, grows weaker?
(a) Light can cross vacuum, whereas sound transmission requires a material medium.
Sound is a mechanical disturbance of matter. The optical signal is electromagnetic and does not need the evacuated air.
At one point and instant, two transverse pulses separately produce displacements +5 mm and -3 mm relative to the same positive direction. What is their resultant displacement?
(b) 2 mm
Apply superposition at the same position and time. Add signed displacements rather than their magnitudes.
Two pulses overlap at a point. Pulse1 alone gives +7 mm, while the measured resultant is +3 mm. What signed displacement must pulse2 alone give?
(a) -4 mm
Write y_total=y1+y2 with signed quantities. Subtract the known contribution from the resultant.
The figure is a spatial snapshot of a sinusoidal progressive wave. P and Q are successive marked zero crossings. Which statement about their oscillations is correct?
(c) Their phase difference is π radians although their instantaneous displacements are equal.
A full wavelength connects repeated phase, including the direction of local motion. Successive zero crossings are half a wavelength apart and differ in phase by π.
A wave is y = 3 sin[36πt + (9/500)πx + π/4], with x and y in cm and t in s. What is its frequency?
(b) 18 Hz
The angular frequency is the coefficient of time in radians per second. Divide it by 2π to obtain frequency in cycles per second.
Two sounds of comparable amplitudes have frequencies 324 Hz and 318 Hz. How many intensity maxima per second does their beat pattern have?
(a) 6 per second
The beat rate is the absolute frequency difference. Both positive and negative extrema of the signed envelope produce intensity maxima.
A string carries transverse waves at 20 m/s. If its tension is multiplied by 4 with linear mass density fixed, what is the new wave speed?
(a) 40 m/s
String speed is sqrt(F/μ). Multiply the old speed by the square root of the tension factor.
A transverse harmonic wave travels right at 40 m/s. A marked string particle is at its maximum upward displacement at this instant. What is its instantaneous transverse velocity?
(c) 0 m/s.
A material particle executes transverse oscillation. At its displacement extremum it reverses motion and its instantaneous velocity is zero.
Two monatomic ideal gases are at the same pressure and temperature. Their densities obey ρ1/ρ2 = 4 and their γ values are equal. What is v1/v2 for sound?
(b) 1/2
The equal pressure and γ give equal adiabatic bulk moduli. Speed ratio is the square root of the inverse density ratio.
An ideal pipe closed at one end and open at the other has fundamental frequency 425 Hz. Neglect end corrections. What is its first overtone?
(a) 1275 Hz
Mixed displacement boundary conditions allow only odd harmonics. The next mode after the fundamental is the third harmonic.
A pulse with upward displacement reaches an ideal free end of a string, such as a frictionless movable ring. How is its displacement orientation after reflection?
(c) It remains upward.
A free endpoint can move transversely. Its displacement reflection has no inversion in the ideal lossless model.
The figure shows one normal mode of a fixed-end string at maximum deformation. What is the phase relation between the oscillations of particles P and Q?
(c) They differ by π radians because they lie in adjacent loops.
The spatial factor changes sign across a node. Particles in adjacent loops oscillate in opposite phase.
In a standing-wave experiment, point P is at zero displacement in one photograph. In later photographs it moves above and below equilibrium. Which classification is justified?
(a) It is not a node, because its displacement does not remain zero.
A node has zero oscillation amplitude and remains at equilibrium at all times. A moving point can pass through zero without being a node.
A string fixed at both ends has fundamental frequency 60 Hz. What is its first overtone frequency?
(a) 120 Hz
The fundamental is the first harmonic. For this string, the next allowed mode is the second harmonic and first overtone.
Wrong
A pulse carries the particles of a string to the far end.
Why it feels right. The bump travels across the string.
Right
The disturbance propagates and each small part of the string moves locally.
Worked example
