Light passing through a prism is mostly refracted, or bent, both when it enters the prism and again when it leaves the prism. Incident light waves tend to be fully reflected from a boundary met at a shallow angle at a certain critical angle and at greater angles, some of the light is also refracted looking at the surface of water from a boat, for instance, one can see down into the water only out to where the sight line reaches the critical angle with the surface. Waves entering a medium with a lower index are accelerated and leave the boundary and enter the second medium at a lesser angle. Waves entering a medium with a higher index of refraction are slowed, leaving the boundary and entering the second medium at a greater angle than the incident wave. The amount of deviation or “bending” depends on the indexes of refraction of each medium, determined by the relative speed of the wave in the two media. Refraction occurs, as in a lens, when a wave passes from one medium into the second, deviating from the straight path it otherwise would have taken. Reflection occurs, as in a mirror, when a wave encounters the boundary but does not pass into the second medium, instead immediately changing course and returning to the original medium, typically reflecting from the surface at the same angle at which it contacted it. The media might consist of two different substances, such as glass and air, or a single substance in different states in different regions, such as air at different temperatures or densities in different layers. The terms refraction and reflection describe two ways that waves, as of sound or light, change course upon encountering a boundary between two media.
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