The key difference lies in the reference medium from which light is traveling.
Understanding Refractive Index
The refractive index is a fundamental optical property of a material. As defined: The refractive index is a measurement of how much a light ray bends when it passes from one medium to another.
When light moves from one substance (like air) into another (like water or glass), its speed changes. This change in speed causes the light ray to bend or change direction. The refractive index quantifies the extent of this bending.
- General Refractive Index: This term can refer to the refractive index of medium 2 with respect to medium 1 (often denoted as $n_{21}$). It describes how light bends when moving specifically from medium 1 to medium 2.
What is Absolute Refractive Index?
The absolute refractive index is a specific case of the refractive index. Based on the definition provided: The refractive index is known as the absolute refractive index when light travels from a vacuum to another medium.
A vacuum is chosen as the reference medium because it represents the conditions under which light travels at its maximum possible speed (the speed of light in vacuum, denoted as c). This makes the absolute refractive index a universal property of a material, independent of the initial medium (as long as it's a vacuum).
- Absolute Refractive Index: This measures how much light bends when it travels from a vacuum into a specific medium (like water or glass). It is often simply referred to as the refractive index (n) of that medium. It is calculated as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v): $n = c/v$.
Comparing Refractive Index and Absolute Refractive Index
Here's a simple comparison:
| Feature | Refractive Index | Absolute Refractive Index |
|---|---|---|
| Starting Medium | Any medium (e.g., Medium 1) | Vacuum |
| Ending Medium | Any different medium (e.g., Medium 2) | Any medium |
| Reference | Medium 1 | Vacuum |
| Typical Denotation | $n_{21}$ (index of medium 2 w.r.t. 1) | $n$ (index of the medium) |
| Calculation Basis | Ratio of speeds in Medium 1 and Medium 2 | Ratio of speed in Vacuum and Medium |
In essence, the absolute refractive index of a material is its refractive index when the light originates specifically from a vacuum. The general term 'refractive index' can refer to the absolute refractive index when the context is clear, or it can refer to the relative refractive index between two non-vacuum media.
For example, the absolute refractive index of water is approximately 1.33. This means light travels 1.33 times slower in water than in a vacuum. The refractive index of water with respect to air is slightly different (around 1.33 too, because air is very close to a vacuum in its optical properties, but not exactly the same).
Understanding the absolute refractive index is crucial as it provides a standard value for how a material affects the speed and bending of light, using the universal speed of light in a vacuum as the baseline.
