When the direction of vibration of the particles of a medium is perpendicular to the direction of propagation of a wave, the wave is said to be--------

Step 1: Analyze the question The question describes the relationship between two directions:

• The direction in which the particles of the medium vibrate
• The direction in which the wave travels (propagates)

Here, the two are perpendicular (at 90° to each other).

Step 2: Explain the concept Waves are classified by how particles vibrate relative to the wave's direction of travel:

| Type | Particle vibration vs. wave direction | Examples | |---|---|---| | Transverse | Perpendicular (90°) | Water waves, light and other electromagnetic waves, waves on a string | | Longitudinal | Parallel (along the same line) | Sound waves, compressions and rarefactions in a spring |

Step 3: Check each option

• A. Longitudinal: Wrong. Here the vibration is parallel to the direction of propagation.
• B. Transverse: Correct. Perpendicular vibration is the definition of a transverse wave.
• C. Mechanical: Wrong. This describes waves that need a medium, and it does not say anything about the vibration direction. Both transverse and longitudinal waves can be mechanical.
• D. A sound wave: Wrong. Sound in air is longitudinal.

Step 4: Answer The wave is transverse, which is option B.

Exam tip: Remember "T = Trans = Across." Transverse waves vibrate across the direction of travel, like a crest and trough on a rope. For longitudinal waves, think of "L = Along", as in compressions and rarefactions along the line of travel.