The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical soda-lime glass has a refractive index of 1.5, which means that in glass, light travels at $ 1/1.5=0.67 $ times the speed The refractive indices which were measured along the dashed line in Figure 1(c) at the radius of 0, 25 mm, 50 mm, 65 mm, 75 mm and plotted in Figure 4 demonstrate that the refractive indices of the film are also approximately consistent with the minority carrier lifetime, the same as the situation discussed previously. Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength of 589 nanometers. There are also weaker dependencies on temperature , pressure / stress , etc., as well on precise material compositions (presence of dopants , etc.); for many materials and typical conditions, however, these variations are at the percent level or less.