Skip to content
Know.itKnow.it
Create an animated lesson
On request

Read the answer free. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

What is the difference between transverse and longitudinal waves?

Class 11 PhysicsNCERT chapter: Wavesइसे हिंदी में पढ़ें

Exam-ready answer

In a transverse wave, the constituents of the medium oscillate perpendicular to the direction of wave propagation, forming crests and troughs, as in a wave on a stretched string. In a longitudinal wave, the constituents oscillate along the direction of propagation, forming compressions and rarefactions, as in sound waves in air. Transverse mechanical waves need a medium with shear modulus.

Explanation

A mechanical wave carries energy through a medium without carrying the medium along. Each particle only oscillates about its mean position and passes the disturbance to its neighbour. The difference between the two types lies in the direction of this oscillation compared with the direction in which the wave travels. Moving one end of a stretched string up and down sends a transverse wave; pushing and pulling a long spring along its length sends a longitudinal wave.

Transverse waves: particles move at right angles to the wave direction. The highest points are crests and the lowest points are troughs. Such waves need the medium to resist a change in shape, that is, to have a shear modulus. So transverse mechanical waves travel in solids and along strings but not through the bulk of liquids or gases. The speed on a string is v = √(T/μ), where T is the tension and μ the mass per unit length.

Longitudinal waves: particles move back and forth along the wave direction, creating compressions (regions of high pressure and density) and rarefactions (regions of low pressure and density). They depend on the bulk modulus, which every medium has, so they travel through solids, liquids and gases. Sound in air is longitudinal. Its speed in a fluid is v = √(B/ρ), where B is the bulk modulus and ρ the density.

Points to remember

  1. Transverse wave: particles oscillate perpendicular to the direction of propagation.
  2. Longitudinal wave: particles oscillate along the direction of propagation.
  3. Transverse → crests and troughs; longitudinal → compressions and rarefactions.
  4. Transverse mechanical waves travel only in solids and strings; longitudinal waves travel in solids, liquids and gases.
  5. Speeds: on a string v = √(T/μ); sound in a fluid v = √(B/ρ).

Exam tip

Present the answer as a two-column table: direction of particle motion, crest/trough vs compression/rarefaction, media in which each travels and one example each. Add a small sketch of each wave.

Questions students also ask

Why can't transverse mechanical waves travel through air?
A transverse wave needs the medium to resist a change in shape, which requires a shear modulus. Gases and liquids have no shear modulus, so they cannot support transverse mechanical waves through their bulk. Air only supports longitudinal waves like sound.
What are compressions and rarefactions?
In a longitudinal wave, a compression is a region where particles are crowded together, so the pressure and density are higher than normal. A rarefaction is a region where they are spread out, so the pressure and density are lower than normal.
Is a sound wave in a solid transverse or longitudinal?
A solid has both bulk and shear moduli, so it can carry both kinds of mechanical waves. Earthquake waves are an example: they travel through the Earth's rocks as both longitudinal and transverse waves, which arrive at different speeds.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.