Light propagation in absorbing materials can be described using a complex-valued refractive index. This effect can be observed in prisms and rainbows, and as chromatic aberration in lenses. This causes white light to split into constituent colors when refracted. The refractive index may vary with wavelength. This implies that vacuum has a refractive index of 1, and assumes that the frequency ( f = v/ λ) of the wave is not affected by the refractive index. The refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is v = c/ n, and similarly the wavelength in that medium is λ = λ 0/ n, where λ 0 is the wavelength of that light in vacuum. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection, their intensity ( Fresnel's equations) and Brewster's angle. This is described by Snell's law of refraction, n 1 sin θ 1 = n 2 sin θ 2, where θ 1 and θ 2 are the angle of incidence and angle of refraction, respectively, of a ray crossing the interface between two media with refractive indices n 1 and n 2. The refractive index determines how much the path of light is bent, or refracted, when entering a material.
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