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The density of a material affects the speed of a wave traveling through it. A seismic wave travels faster through more dense material than through material that is less dense. The layers of material in the Earth often have different densities. A seismic wave traveling through these different layers might travel at several different speeds by the time it arrives at a detector.

The density of a material can also affect the direction of an incoming wave. If a seismic wave traveling through low-density material enters a layer of higher density material at an angle other than 90 degrees, the wave is bent away from the boundary where the two layers meet.

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If a wave travels through high-density material and enters lower density material at an angle of other than 90 degrees, the wave is bent toward the layer boundary. This change of direction or bending is called refraction. It is seen in many types of waves, including light, water, and sound waves.

If a wave enters a layer of material at an angle of 90 degrees, the wave does not bend, regardless of any differences in density. However, the speed of the wave will still change, depending upon the density of the material.

If there is a sudden change in density, the wave will suddenly change direction. If the increase in density is gradual, the bending will be gradual. A very thick layer of rock gradually increases in density from top to bottom as pressure increases with depth. A wave moving down through this layer would gradually increase its speed.

You will experience refraction of light as you stand at the edge of a swimming pool and look at an object on the bottom. The object will look closer to you, and further from the edge of the pool, than it actually is. This is due to the bending of the light as it passes from water to air. Water and air have different densities, so the light passes through them at different speeds.

Measurements of refracted waves can give us valuable information. Sound wave refraction is used in oil exploration to improve the accuracy of seismic surveys. Refraction data can identify features such as salt domes. These underground deposits of salt are important to oil exploration because hydrocarbons are commonly found around salt domes since salt movements create abundance and variety of traps for oil, and the association with evaporite minerals that provides excellent sealing capabilities.

Refraction of light has many applications, such as the lenses of spectacles, which use refraction to modify the image of an object, such as magnification.