How this calculator works
The metre is defined using the exact vacuum speed of light. Multiplying speed by travel time gives distance; dividing distance by speed gives time. The medium option treats index as a constant. For timing a pulse in a dispersive material, enter the group index rather than the phase index. The calculation omits cosmological expansion, gravity and moving endpoints.
Formula
c = 299792458 m/s; v = c/n; d = vt; t = dn/c. A light-year uses a Julian year of 365.25 days.
Worked example
Light travels 299792458 m in one second in vacuum. The same distance takes two seconds if the effective timing index is 2.
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Frequently asked questions
Is a light-year a time unit?
It is a distance: the distance light travels in vacuum during one Julian year.
Is the vacuum value measured approximately?
Its SI value is exact by definition. Experimental distance and time measurements still have uncertainty.
Does this give a radio echo delay?
For an echo, use the full outward and return distance, which is twice the one-way range for stationary endpoints.