Absolute & Relative Refractive Index
Definition of Refractive Index
What is Refractive Index? The definition of refractive index can be divided into the following two types, absolute refractive index and relative refractive index:
Absolute Refractive Index
Refractive Index is the ratio of speed of light in vacuum to the speed of light in medium, also known as "Absolute Refractive Index", which is defined:
η = c / v (η is pronounced Eta)
In the above formula, c is the speed of light in vacuum, and v is the speed of light in medium.
The speed of light mentioned here for refractive index refers to "Phase Velocity", instead of "Group Velocity". Phase velocity refers to the speed at which a certain phase of the light wave (e.g. wave crest) moves from point A to point B. Group velocity refers to the speed of a light wave (or a wave packet), which passing through a fixed point with a complete period.
From the below animation (refers to Wikipedia), you can understand the difference between phase velocity and group velocity. The red point moves relative to the picture, which is the phase velocity of the wave; the green point is stationary, and the speed of wave passes with a complete wave period through the green point is called the group velocity.
The following conclusions can be drawn from the above formula:
A medium with a high absolute refractive index through which light travels slowly. A medium with a low absolute refractive index through which light travels quickly. refractive index vs.
Relative Refractive Index
If you want to describe the relative relationship between the refractive index of two non-vacuum media, you can use "relative refractive index" to express:
η21 = η2 / η1 = v1 / v2. That is to say, the absolute refractive index of medium 1 is used as a benchmark to compare the refractive index multiple of medium 2.
For example, medium 2 is air with a refractive index of 1.0003, and medium 1 is pure water with a refractive index of 1.333. Then the relative refractive index of air relative to pure water is 1.0003 / 1.333 = 0.75. That is to say, the refractive index of pure water in medium 1 is used as the benchmark, and compared with air in medium 2, the refractive index of air is only 0.75 times that of pure water. If the above description is written as a formula, we can get:
η21 = η2 / η1 = (c / v2 air) / (c / v1 water) = v1 water / v2 air
The meaning of absolute refractive index
Let's come back to the absolute refractive index topic mentioned earlier, we can understand the absolute refractive index as the refractive index (1.0000) of a certain dielectric material relative to vacuum, which is the refractive index based on the vacuum refractive index:
η = η material / η vacuum, the denominator is the vacuum refractive index, which is 1.0000
Or use the speed of light to calculate:
η = c / v material
To obtain the refractive index of a certain medium, you can use the interference method to measure the speed of light in the medium, and then divide the speed of light in vacuum (299792458 m/s) by the speed of light in the medium to get the absolute refractive index of the medium.
What Are the Measurement Results of a Refractometer?
Refractometers on the market measure the relative refractive index of a medium compared to air, expressed in units of nD.
For example, the refractive index of pure water at 20°C is 1.33336. When the temperature rises to 30°C, the refractive index decreases to 1.33230, and at 40°C, it further decreases to 1.33095. These refractive index values are based on yellow light from a sodium-vapor lamp with a central wavelength of 589 nm.
If a helium-neon laser with a wavelength of 632.8 nm (red light) is used instead, the refractive index of pure water at 20°C changes to 1.33211, which is slightly lower.
Therefore, when measuring or describing the refractive index of gases and liquids, temperature considerations are usually included. The standard refractive index of liquids is typically defined at 20°C, while the standard refractive index of gases is defined at 0°C.
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