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When light travels at an angle between two materials, it bends according to their refractive indices. In order to reflect, light must be on the wider side of the critical angle. The refractive index can provide information for us about the behavior of light. When light passes through the different substances its velocity decreases by increasing of the refractive index of these substances. It can be due to interaction between molecules of components in substrate and effect of these interactions on light. Also in the most substrates, the refractive index decreases by increasing of the temperature. The interaction between molecules decreases as the temperature increases. The refractive index of different substrates measures with refractometers. There are four main types of refractometers: traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometers, and inline process refractometers. There is also the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases.A sodium lamp may be used to provide the light source at a known wave-length (589.6 nm) although many instruments are corrected for daylight use. In most of substrates which we know the refractive index have a positive value (more than zero) but in 1968 Veselago shows that there are substrates with negative permittivity and negative permeability. In these substrates refractive index have a negative value.
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