Light is fast but not instant: it takes 1.3 s to reach the Moon, 8 min 20 s from the Sun and about 4.2 years from the nearest star. Signals in optical fibres travel about a third slower because of the glass’s refractive index.
How to use the Speed of Light Calculator
- Find
- Refractive index of the medium
- Time
- Distance
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose whether you want the distance or the time, and the medium’s refractive index (1 for vacuum).
Inputs explained
| Field | What to enter |
|---|---|
| Find | Choose one: Distance, Time |
| Refractive index of the medium | A number |
| Time | Units: s, ms, min, h, days |
| Distance | Units: km, m, mi, au, ly |
What each result means
- Speed of light in the medium = 299792458÷[Refractive index of the medium]
- Distance = 299792458÷[Refractive index of the medium]×[Time]
- Time = [Distance]×[Refractive index of the medium]÷299792458
Example values
How the main result changes when Refractive index of the medium changes and everything else stays at the example values above:
| Refractive index of the medium | Distance (km) |
|---|---|
| 1 | 299,792.458 |
| 1.25 | 239,833.9664 |
| 1.5 | 199,861.6387 |
| 2 | 149,896.229 |
Formula
d = (c/n)·t, c = 299,792,458 m/s
Worked example
Inputs — Find: Time; Refractive index of the medium: 1; Distance: 384,400 km.
Results — Speed of light in the medium = 299,792.458 km/s; Time = 1.28222 s.
Key terms
- Speed of light
- c = 299,792,458 m/s.
Frequently asked questions
How fast is light?
Exactly 299,792,458 m/s in vacuum.
How long does sunlight take to reach Earth?
About 8 minutes 20 seconds.
Is light slower in glass?
Yes – about 200,000 km/s in typical glass.
Why is c exact?
Since 1983 the metre has been defined from the speed of light.