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Refraction is a change in the direction of electromagnetic radiation (EMR) passing from one medium to another, such as visible light passing from air into glass or glass to air. It is a general property of waves and occurs with sound or ocean waves as well as EMR at points where the speed of the wave changes. The common use of the term is for the redirection of light passing through some substance in which light travels slower, such as glass or water. Travel between the interplanetary medium (IPM) and Earth's atmosphere also produces it. Refraction through a curved boundary is the basis for the magnification of refracting telescopes and microscopes. The affect of such refraction is described by Snell's law, which relates the amount of redirection to the difference in the properties of the two media, each medium's property termed its refractive index.