Library / Foundations: Physics / FPH-35
Foundations: Physics
Refractive index
Ratio of the speed of light in vacuum to its speed in a medium.
- Ref
- FPH-35
- Rev
- v1
- Field
- Foundations: Physics
- Result
- n
c = 2.998×10⁸ m/s is the speed of light in vacuum and v the speed in the medium.
Light slows down in materials; the refractive index tells by how much.
Calculator
10⁸ m/s
- Fractional slowdown33.29 %
- Wavelength relative to vacuum0.6671
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
Refractive index () current value 1.499
vSpeed in mediumKey driver
1.032.5
- Glass has n of about 1.5, so light travels at about two thirds of its vacuum speed.
- Frequency stays the same in a medium; the wavelength shortens.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing () | Local step | Change in result |
|---|---|---|---|---|---|---|
| v Speed in medium | 1.2 to 2.9 10⁸ m/s | 1.034 | 2.498 | 1.465 | +10% | −9.09 % |
Sweep of Speed in medium
| Speed in medium (10⁸ m/s) | Refractive index |
|---|---|
| 1.2 | 2.498 |
| 1.412 | 2.122 |
| 1.625 | 1.845 |
| 1.837 | 1.632 |
| 2.05 | 1.462 |
| 2.263 | 1.325 |
| 2.475 | 1.211 |
| 2.688 | 1.116 |
| 2.9 | 1.034 |
Sweep one variable
Refractive index ()Hover, touch, or use the arrow keys on the chart
Variables and how each one contributes
n
Refractive index
What the equation gives you. Everything else feeds it.
v
Speed in medium
Speed of light in the material. Inversely proportional.
Worked example with the values above
- Speed in medium: 2×10⁸ m/s.
- n = c/v = 2.998×10⁸ / 2×10⁸.
- n = 1.499.
Assumptions and limits
- Uniform medium.
- Fixed wavelength.
Common mistakes
- Inverting the ratio.
- Confusing n with density.
Statistics and uncertainty
The speed is not measured directly; n is usually found from angles, so errors follow from the angle readings.
Where it is used
- Lens and window materials.
- Fibre optic delay estimates.
Maths you need
- Speed of light: c = 2.998×10⁸ m/s
- Ratios: n = c/v