Examples to include: sound, electromagnetic waves, and waves on a string.

The idea

Waves transfer energy without transferring matter, and the three named examples let you classify motion by how particles or fields oscillate. Sound in air is longitudinal: air molecules vibrate parallel to the direction the sound travels, forming compressions and rarefactions. Electromagnetic waves are transverse: oscillating electric and magnetic fields are perpendicular to the direction of energy transfer, and they need no medium. Waves on a string are transverse: each small element of string moves up and down while the wave pattern moves along the string. To classify an example, identify the oscillating quantity, then compare its direction with the direction of energy propagation. If those directions are parallel the wave is longitudinal; if perpendicular it is transverse.

What earns the marks

• Sound is longitudinal because the oscillations of the particles are parallel to the direction of energy transfer.
• Electromagnetic waves are transverse because the electric and magnetic field oscillations are perpendicular to the direction of energy transfer.
• Waves on a string are transverse because the displacement of string elements is perpendicular to the direction of energy transfer along the string.
• A valid classification must name the oscillating quantity and compare its direction with the direction of energy propagation.
• Electromagnetic waves are the example that does not require a material medium, unlike sound and waves on a string.

Common mistakes and how to put them right

• Calling sound transverse because it can be reflected or refracted.

  Put it right: Classify by the direction of particle oscillation relative to energy transfer, so sound in a fluid is longitudinal.

• Saying electromagnetic waves are longitudinal because they can travel through a vacuum.

  Put it right: The absence of a medium does not determine wave type, and electromagnetic waves are transverse.

• Describing a wave on a string as longitudinal because the string is stretched along its length.

  Put it right: The tension is along the string, but the displacement of each element is perpendicular to the energy transfer.

Exam tips

• For each example, state the oscillating quantity and then compare its direction with the direction of energy transfer.
• Use the words parallel and perpendicular explicitly; vague phrases such as 'side to side' do not show the required comparison.
• When asked for examples, give the three named in the specification and add the classification for each rather than listing them alone.