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Snell’s law governs the imaging properties of lenses. In the special case where the incident beam is perpendicular to the boundary (that is, equal to the normal), there is no change in the direction of the light as it enters the second medium. Conversely, light emerging obliquely from a denser medium is bent away from the normal. When light passes into a denser medium, the ray is bent toward the normal. The amount of bending of a light ray as it crosses a boundary between two media is dictated by the difference in the two indices of refraction. The basic features of refraction are easily derived from Snell’s law. For example, the index of refraction of air at standard conditions is 1.0003, water is 1.33, and glass is about 1.5. Because the speed of light in any transparent medium is always less than the speed of light in a vacuum, the indices of refraction of all media are greater than one, with indices for typical transparent materials between one and two. © MinutePhysics ( A Britannica Publishing Partner) See all videos for this articleīy definition, the index of refraction for a vacuum is exactly 1. Learn about refraction and how the speed of light changes in glass.
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The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium.
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The law of refraction, also known as Snell’s law, describes the relationship between the angle of incidence (θ 1) and the angle of refraction (θ 2), measured with respect to the normal (“perpendicular line”) to the surface, in mathematical terms: n 1 sin θ 1 = n 2 sin θ 2, where n 1 and n 2 are the index of refraction of the first and second media, respectively. As the transmitted light moves into the second medium, it changes its direction of travel that is, it is refracted. When light traveling in one transparent medium encounters a boundary with a second transparent medium (e.g., air and glass), a portion of the light is reflected and a portion is transmitted into the second medium. This diffuse reflection enables illuminated objects to be seen from almost any line-of-sight location. When light strikes rough surfaces, it reflects at many angles. Diffuse reflection is responsible for the ability to see most illuminated surfaces from any position-rays reach the eyes after reflecting off every portion of the surface. Unlike mirrors, most natural surfaces are rough on the scale of the wavelength of light, and, as a consequence, parallel incident light rays are reflected in many different directions, or diffusely. The law of reflection can be used to understand the images produced by plane and curved mirrors. (By convention, all angles in geometrical optics are measured with respect to the normal to the surface-that is, to a line perpendicular to the surface.) The reflected ray is always in the plane defined by the incident ray and the normal to the surface. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray. Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing.