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This important law, named after Dutch mathematician Willebrord Snell, states that the product of the refractive index and the sine of the angle of incidence of a ray in one medium is equal to the product of the refractive index and the sine of the angle of refraction in a successive medium. Also, the incident ray, the refracted ray, and the normal to the boundary at the point of incidence all lie in the same plane. Generally, the refractive index of a denser transparent substance is higher than that of a less dense material; that is, the velocity of light is lower in the denser substance. If a ray is incident obliquely, then a ray entering a medium with a higher refractive index is bent toward the normal, and a ray entering a medium of lower refractive index is deviated away from the normal. Rays incident along the normal are reflected and refracted along the normal. In making calculations, the optical path, which is defined as the product of the distance a ray travels in a given medium and the refractive index of that medium, is the important consideration. To an observer in a less dense medium such as air, an object in a denser medium appears to lie closer to the boundary than is the actual case. A common example, that of an object lying underwater observed from above water, is shown in Fig. 3. Oblique rays are chosen only for ease of illustration. The ray DB from the object at D is bent away from the normal to A. The object, therefore, appears to lie at C, where the line ABC intersects a line normal to the surface of the water and passing through D.

The path of light passing through several media with parallel boundaries is shown in Fig. 4. The refractive index of water is lower than that of glass. Because the refractive index of the first and last medium is the same, the ray emerges parallel to the incident ray AB, but it is displaced.

Until 2001, all known substances had a positive refractive index. In that year physicist Sheldon Schultz and his colleagues at the University of California at San Diego created a composite from fiberglass and copper wire that refracts microwaves in the direction opposite that in which all other materials refract light. This unusual refraction indicates that the material has a negative refractive index. Since microwaves, like visible light, are a type of electromagnetic radiation, scientists predict that it will eventually be possible to produce a material that refracts visible light in the same way.