Light bends when it travels from a certain density of matter into a different density of matter. The density of the stuff affects how much it bends. Light bends when it transitions from air to water or from water to air. Light bends when it travels from water to glass or from glass to air. How much light is bent by a particular substance may be determined by its index of refraction. However, Snell's Law and the angle of incidence must be coupled. The angle of refraction increases with increasing angle of incidence. As a result, the water's unique index of refraction bends light entering the medium at an angle of 30°. and that is 1.33. This means that light in a vacuum travels at 1.33 times faster than light in water. We wouldn't have lenses in the way we do now without refractive index. We would need eyes with pinhole-sized openings in order to see, which would prevent us from seeing clearly or at least in great detail. We wouldn't have had microscopes to view anything in great detail, telescopes to view the moon, planets, or distant things. I could go on, but I think you get the idea: if we didn't have the material characteristic known as refraction, things would be quite different. Well, I guess it's possible that human eyes have evolved to have diffractive lenses, but that's another theory.
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The ability of a substance, whether it is solid, liquid, or gas, to reflect light causes it to move more slowly than it would in a vacuum.
Any substance's refractive index (n) is found by dividing the speed of light in that substance by the speed of light in a vacuum (c) (v).
So
n = c/v
Since a material's refractive index varies depending on the wavelength of light, n = f(wavelength) (n is a function of wavelength). In comparison to longer wavelengths, the index is larger for shorter wavelengths. It's known as dispersion.
When creating lenses or other refractive optical systems, the refractive index and dispersion are both crucial factors. The diverse wavelengths don't come to a common focus, which has a disastrous effect on image quality because index is directly related to how light bends while passing through a lens. We refer to this as chromatic aberration. It's difficult to regulate, but you can achieve it by carefully selecting various glass chemistries and massaging them into position. The glass map showing index as a function of inverse dispersion is seen in the image below. About 20 to 100 is the range of the Abby number, a measure of dispersion; lower numbers indicate more dispersion (larger index difference between red and blue light). Every dot stands for a distinct glass.
Refractive indices range from 1.0003 for air, to over 4.5 for Geranium.