Waves transmit energy, but not all waves are alike. We can categorize waves in several ways. First, we can categorize waves on the basis of the direction of movement of the individual particles of the medium relative to the direction that the waves travel. A transverse wave is a wave in which particles of the medium move in a direction perpendicular to the direction that the wave moves. Wave energy will begin to be transported from left to right, but the medium will be displaced upwards and downwards. Electromagnetic waves are transverse waves. The parts of these waves include the crest, or peak, and the trough, or valley. A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction that the wave moves. As the energy is transported from left to right, the medium will be displaced left and right as well, so longitudinal waves are always characterized by particle motion being parallel to wave motion. Sound waves are an example of a longitudinal wave. The main parts of a longitudinal wave are the areas of compression and rarefaction.

Another way to classify these waves is how they travel or are transmitted. Electromagnetic waves can travel through a vacuum; sound waves cannot. The speed of these two waves also varies with the medium they are passing through. A sound wave travels fastest through a solid because the particles are closest together in this state of matter and easier to compress back and forth. An electromagnetic waves travels fastest through a vacuum and slowest through a solid. An electromagnetic wave is formed by the vibrations of electric and magnetic fields. Electromagnetic waves are made of discrete packets of energy called photons; therefore these waves have both particle-like and wave-like properties.

How do mechanical waves compare with electromagnetic waves?

The letter D. in the image refers to the crest of a wave.
This is a sound wave.
Wave energy is transported from left to right.
Can you hear the train coming? You put your ear to the ground because sound travels fastest through a solid.
Letter C. is an area of compression.