Free · Physics models
Absolute and Relative Refractive Index
Compare vacuum-to-medium phase-speed ratios or the ratio of two entered medium speeds.
- Formula & worked example
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Calculator inputs
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How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
Private by default
Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.
Formula and method
A custom phase speed of 149,896.229 kilometers per second is half the exact vacuum light speed, giving an absolute refractive index of 2.
Absolute index = vacuum light speed/medium phase speed. The relative index of medium 2 with respect to medium 1 is speed 1/speed 2.
Worked example
Enter these known values and leave the other values blank.
- Medium
- Custom
- Speed in medium (v)
- 149896.229 km/s
- Index of refraction
- 2
Assumptions and limitations
- Vacuum light speed is exactly 299,792,458 m/s. The absolute mode uses positive medium speed no greater than this constant; a relative ratio may be below one.
- The material menu retains rounded illustrative phase-speed presets. Refractive properties depend on wavelength, temperature and composition; use Custom for a measured phase speed.
- These speed ratios do not calculate a refracted ray angle by themselves. That additionally requires the incident angle and the boundary relation; absorption, anisotropy and dispersion are not modeled.
- Clear the unknown numeric field before solving a supported inverse.
Common questions
What does this scenario assume?
Vacuum light speed is exactly 299,792,458 m/s. The absolute mode uses positive medium speed no greater than this constant; a relative ratio may be below one.
What does the result leave out?
These speed ratios do not calculate a refracted ray angle by themselves. That additionally requires the incident angle and the boundary relation; absorption, anisotropy and dispersion are not modeled.
References
- NIST: exact vacuum speed of light in the SI
- OpenStax: refractive index as a phase-speed ratio
- Calculation definition and unit reference
Model input requirements: Absolute-mode medium speed must be positive. Relative-mode first-medium speed must be positive. Relative-mode second-medium speed must be positive. Original accessible descriptions replace the two captured refraction diagrams and preserve their absolute/relative speed-ratio explanations in the corresponding mode.