Which is a longitudinal wave?

Which is a longitudinal wave?

• A. Light
• B. Radio waves
• C. Sound
• D. Water waves

Answer: (C)

The key idea is how the medium’s particles move relative to the direction the wave travels. A longitudinal wave has particle motion in the same direction as the wave’s travel. Sound fits this: as it moves through air, air molecules compress and then spread apart along the direction the sound is moving, creating compressions and rarefactions that propagate forward. Light and radio waves are electromagnetic and are transverse: their oscillations are perpendicular to the direction of travel, and they can move through a vacuum. Water surface waves aren’t pure longitudinal waves either; the water particles move mainly up and down as the wave passes, with some sideways motion, so they’re not a simple longitudinal wave. So the option that is a longitudinal wave is sound.

The key idea is how the medium’s particles move relative to the direction the wave travels. A longitudinal wave has particle motion in the same direction as the wave’s travel. Sound fits this: as it moves through air, air molecules compress and then spread apart along the direction the sound is moving, creating compressions and rarefactions that propagate forward.

Light and radio waves are electromagnetic and are transverse: their oscillations are perpendicular to the direction of travel, and they can move through a vacuum. Water surface waves aren’t pure longitudinal waves either; the water particles move mainly up and down as the wave passes, with some sideways motion, so they’re not a simple longitudinal wave.

So the option that is a longitudinal wave is sound.