Waves — Class 11 MCQs with Answers

Waves — important questions & MCQs with answers (Class 11 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Light is a ___ wave.

   ATransverseBLongitudinalCMechanicalDStationary only
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   Answer: Transverse

   Explanation: Light is an electromagnetic wave: E and B oscillate perpendicular to the direction of travel, making it transverse. It needs no medium, unlike mechanical waves such as sound — that's the tempting mix-up here.

2. Q2 of 6Easy

   Transverse wave: particle motion is:

   AParallel to wave propagationBPerpendicular to wave propagationCRandomDNo motion
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   Answer: Perpendicular to wave propagation

   Explanation: In a transverse wave, particles oscillate perpendicular to the direction the wave travels — like a string moved up and down while the wave moves sideways. Parallel motion instead describes a longitudinal wave, the usual mix-up.

3. Q3 of 6Medium

   Wave with f = 100 Hz, λ = 5 m. Speed:

   A105 m/sB500 m/sC20 m/sDCannot compute
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   Answer: 500 m/s

   Explanation: Wave speed v = fλ = 100 Hz × 5 m = 500 m/s. Picking 105 m/s (adding instead of multiplying) or 20 m/s (dividing) confuses the multiplicative relation between frequency and wavelength.

4. Q4 of 6Medium

   Speed of sound in gas: v = ?

   A√(γRT/M)B√(P/ρ)C√(γM/RT)DγRT/M
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   Answer: √(γRT/M)

   Explanation: Speed of sound in a gas is v = √(γRT/M) (equivalently √(γP/ρ) via the ideal gas law) — Laplace's correction to Newton. √(P/ρ) is Newton's isothermal formula, which leaves out γ and gives about 280 m/s in air; √(γM/RT) is inverted, and γRT/M is missing the square root.

5. Q5 of 6Hard

   Wave moving in +x direction: y(x, t) = ?

   A2A sin(kx) cos(ωt)BA sin(ωt + kx)CA sin(ωt − kx)DA sin(ωt)
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   Answer: A sin(ωt − kx)

   Explanation: A wave written as y = A sin(ωt − kx) moves in the +x direction, since x must increase as t increases to keep the phase (ωt − kx) constant. The form A sin(ωt + kx) instead describes a wave moving in the −x direction, 2A sin(kx) cos(ωt) is a standing wave, and A sin(ωt) has no x-dependence at all.

6. Q6 of 6Hard

   When two equal waves traveling in opposite directions meet:

   AStanding wave is formedBTravelling waveCNothingDCannot say
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   Answer: Standing wave is formed

   Explanation: When two waves of equal amplitude and frequency travel in opposite directions and superpose, their interference locks into a stationary pattern of fixed nodes and antinodes — a standing wave. Calling the result a travelling wave misses that the pattern no longer moves.

Waves notes: key concepts (Class 11 Physics)

How disturbances travel and interact: transverse and longitudinal waves, the wave parameters and the progressive wave equation, wave speed on a stretched string and sound speed in a gas with Laplace's correction, the superposition principle and the three phenomena that follow from it — interference, standing waves and beats — and the Doppler effect.

Wave motion

A disturbance that transports energy and momentum through a medium without any net transport of the medium itself; the particles only oscillate about fixed positions.

Transverse waves

Particles vibrate perpendicular to the direction of propagation. They can travel only through solids and along liquid surfaces, and can be polarised.

Longitudinal waves

Particles vibrate along the direction of propagation, creating compressions and rarefactions. Sound in air is longitudinal and cannot be polarised.

Wave parameters

Amplitude sets the intensity, wavelength is the distance between successive in-phase points, and frequency is fixed by the SOURCE, not by the medium.

Wave speed

Equal to frequency times wavelength, and determined entirely by the MEDIUM. When a wave crosses into a new medium the frequency stays the same and the wavelength changes.

Progressive wave equation

y = A sin(ωt − kx) describes a wave travelling in the positive x-direction; a plus sign before kx reverses the direction of travel.

Wave number and angular frequency

The wave number k is 2π/λ, measuring phase change per unit distance, while ω is 2π/T, measuring phase change per unit time. Their ratio is the wave speed.

Speed on a stretched string

Given by √(T/μ), so tightening a string or using a thinner one raises the speed — the basis of tuning every stringed instrument.

Speed of sound in a gas

Newton's isothermal formula gave a value about 15% too low; Laplace corrected it by treating the compressions as ADIABATIC, giving v = √(γP/ρ).

Temperature dependence of sound

The speed of sound in a gas is proportional to the square root of absolute temperature and is independent of pressure at fixed temperature.

Principle of superposition

When two waves overlap, the resultant displacement at every point is the vector sum of the individual displacements. Everything below follows from this.

Interference

Coherent waves reinforce where the path difference is a whole number of wavelengths and cancel where it is an odd number of half-wavelengths.

Standing waves

Formed when two identical waves travel in opposite directions, producing fixed nodes of zero amplitude and antinodes of maximum amplitude. No energy is transported.

Nodes, antinodes and harmonics

Adjacent nodes are half a wavelength apart. A string fixed at both ends supports only frequencies that are whole-number multiples of the fundamental.

Beats

The slow throbbing heard when two waves of slightly different frequency superpose; the beat frequency is simply the difference of the two frequencies.

Doppler effect

The apparent change in frequency when source and observer move relative to each other. Approaching raises the pitch and receding lowers it.