Q.A pulse travels along a stretched horizontal string, and a sound pulse travels along a column of air in a pipe. State, with a reason for each, whether the wave in the string and the wave in the air column is transverse or longitudinal.

Progressive waves split into two kinds based on how a medium's particles vibrate relative to the wave's own direction of travel. In a transverse wave, particles vibrate PERPENDICULAR to the propagation direction -- water-surface waves are the everyday example. As a transverse wave passes, the medium divides into alternating crests (positive-displacement regions) and troughs (negative-displacement regions); a crest and its neighbouring trough form one cycle. Transverse waves need the medium to have elasticity of SHAPE, so they can travel through solids and along liquid surfaces, but NOT through the bulk of liquids or through gases (electromagnetic waves are the sole exception, needing no medium at all). Because a transverse wave's vibration can be confined to a single plane, it CAN be polarised; pressure and density in the medium stay unchanged as it advances -- only shape changes.

In a longitudinal wave, particles vibrate PARALLEL to the propagation direction -- sound is the standard example. The medium instead divides into alternating compressions (crowded, high-pressure regions) and rarefactions (spread-out, low-pressure regions), with pressure and density genuinely varying both in space and time as the wave advances. Longitudinal waves need elasticity of VOLUME, so they travel through solids, liquids AND gases, but never through vacuum. Because their vibration direction already coincides with their direction of travel, longitudinal waves can NEVER be polarised.

This classification is introduced in the NCERT Class 11 Physics chapter on Waves and is tested regularly in CBSE board papers, JEE Main and NEET alike. It answers exactly the search intent behind "transverse and longitudinal waves difference class 11" or "types of mechanical waves important questions".