Transverse And Longitudinal Waves

The distinction between wave types in NCERT Class 11, Chapter 15 (Waves) rests on the direction of particle vibration relative to propagation. In a longitudinal wave the medium's particles oscillate back and forth parallel to the direction the wave travels, creating alternating compressions and rarefactions. Sound in air does exactly this, so it is a longitudinal mechanical wave requiring a material medium. The option of a transverse mechanical wave is wrong because transverse waves have particle motion perpendicular to travel, as on a stretched string, not the compressions seen in a gas. The option of a transverse electromagnetic wave is wrong because sound needs a medium and is not electromagnetic. The option of a stationary wave is wrong since the described wave clearly propagates along the tube rather than forming fixed nodes. A consistency check: gases cannot sustain shear, so they support only longitudinal (pressure) waves, which is precisely why sound in air must be longitudinal, matching the description of compressions and rarefactions.