Awave can be described as a disturbance that travels through a medium from one location to another location. 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. electromagnetic waves are transverse waves. 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; mechanical waves cannot. a mechanical wave is a wave that is not capable of transmitting its energy through a vacuum. they require a medium in order to transport their energy from one location to another. a sound wave is an example of a mechanical wave. the speed of these two waves, light and sound, 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. can you tell the difference between electromagnetic and mechanical waves? in a mechanical or compression wave, the particles of the medium are displaced in a direction perpendicular to the direction of energy transport. a wavelength: in a transverse wave it can be measured from crest to crest; in a mechanical wave, from compression to compression. a thunderstorm! lightning and thunder and rain! you hear the thunder before you see the lightning because sound travels faster than light through the air. dolphins slap the wave to send messages because sound travels faster through water than through air. sound waves are transferred by the compression of particles. the closer the particles are to one another, the faster the energy is transferred. therefore, sound energy directly depends upon the density of the medium. signals traveling through space must be in the form of electromagnetic waves rather than sound waves.