Waves are a propagation of a disturbance in a medium that transmits energy from one location to another. There are many different types of waves, including sound waves, surface waves, seismic waves and electromagnetic and light waves.
There are two main ways in which the medium oscillates with respect to the direction of wave propagation: transversely and longitudinally. Here are examples of both types of waves and the physics behind them.
Transverse wave motion occurs when points in the medium oscillate at right angles to the direction of the wave's travel. If a wave travels from left to right, the particles of the medium will vibrate up and down in place.
Examples of transverse waves include waves on a string, or the wave created by a stadium crowd at a ball game (where the wave moves around the stadium while the individual people simply move up and down in their seats).
Electromagnetic waves, or light waves, are transverse. As they propagate, electric and magnetic fields oscillate perpendicular to the direction of motion. Electromagnetic waves, unlike mechanical waves which require a medium, can propagate in the vacuum of space. This is how Earth can receive sunlight and starlight from billions of light years away.
Another transverse wave example is the membrane of a drum when struck; the membrane moves up and down as waves travel across its surface. When the membrane vibrates like this, it creates sound waves that propagate through the air, which are longitudinal rather than transverse.
Longitudinal waves, also called pressure waves and compressional waves, oscillate parallel to the direction of the motion of the wave. If a wave travels from left to right, the displacement of the medium is also left to right but oscillating in place by compressing and stretching.
When a sound travels through a medium, such as air, it causes compressions (regions of increased density) and rarefactions (regions of decreased density) in the air as it travels. Different types of sounds will have different patterns of compressions and rarefactions.
Examples of longitudinal waves include sound waves, or the waves you can create by stretching a slinky along the floor and pushing or pulling it along its length. You can also see these waves in water and other fluids or the earth – but these examples can have transverse waves, too.
Water Waves and Earthquakes
Water and earth are mediums that experience both transverse and longitudinal waves. Earthquakes have both types of waves, while water waves have a special combination of transverse and longitudinal which are called surface waves.
In seismology, the study of earthquakes, there are two main types of waves: p-waves and s-waves. P-waves, or primary waves, are longitudinal, where the earth compresses and stretches in the direction of wave propagation.
S-waves, or secondary waves, are transverse, where the earth moves up and down as the wave travels. Primary waves are faster than secondary waves and so are often the first to be detected in an earthquake.
In water and other fluids, waves are more complicated. Water particles on the surface will move up and down as a wave passes – transverse motion – but they will also experience compression at the top of the wave and rarefaction at the bottom – longitudinal motion. In fact, if you follow a single water particle on the surface as waves ripple past it, you will actually see it moving in a circle! These are called surface waves.
Away from the surface, all waves in fluid are longitudinal since bulk fluids are not rigid enough mediums to transmit energy transversely. Bulk solids, however, can sustain transverse waves.