refraction

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Bending of a wave when it passes from one medium into another. It is the effect of the different speeds of wave propagation in two substances that have different densities. For example, when light passes from air (less dense) into glass (more dense) it slows down (from 300 million to 200 million metres per second) and is refracted. The amount of refraction depends on the densities of the media, the angle at which the wave strikes the surface of the second medium, and the amount of bending and change of velocity corresponding to the wave's frequency (dispersion). Refraction occurs with all types of progressive waves – electromagnetic waves, sound waves, and water waves – and differs from reflection, which involves no change in velocity.

Refraction of light
The degree of refraction depends in part on the angle at which the light hits the surface of a material. A line perpendicular to that surface is called the normal. The angle between the incoming light ray and the normal to the surface is called the angle of incidence. The angle between the refracted ray and the normal is called the angle of refraction. The angle of refraction cannot exceed 90°. An example of refraction is light hitting a glass pane. When light in air enters the denser medium, it is bent toward the normal. When light passes out of the glass into the air, which is less dense, it is bent away from the normal. The incident light will be parallel to the emerging light because the two faces of the glass are parallel. However, if the two faces are not parallel, as with a prism, the emerging light will not be parallel to the incident light. The angle between the incident ray and the emerging ray is called the angle of deviation. The amount of bending and change in velocity of the refracted wave is due to the amount of dispersion corresponding to the wave's frequency, and the refractive index of the material. When light hits the denser material, its frequency remains constant, but its velocity decreases due to the influence of electrons in the denser medium. Constant frequency means that the same number of light waves must pass by in the same amount of time. If the waves are slowing down, wavelength must also decrease to maintain the constant frequency. The waves become more closely spaced, bending toward the normal as if they are being dragged. The refractive index of a material indicates by how much a wave is bent. It is found by dividing the velocity of the wave in the first medium by the velocity of the wave in the second medium. The absolute refractive index of a material is the velocity of light in that material relative to the velocity of light in a vacuum. See also apparent depth.

Refraction of sound
Sound waves, unlike light waves, travel faster in denser materials, such as solids and liquids, than they travel in air. When sound waves enter a solid, their velocity and wavelength increase and they are bent away from the normal to the surface of the solid.

Water waves
Water waves are refracted when their velocity decreases. Water waves slow down as water becomes shallower. A good example of this is a wavefront approaching a shore that is shallower in one place and deeper in another. When the wavefront approaches, the part of the wave in shallower water will slow down and its wavelength will decrease, causing it to lag behind the part of the wave in the deeper water.

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