Supposed that a light source
- emits
waves during the proper time interval , which is the time observed from the moving light source - For
in the light source’s frame, the stationary observer experiences coordinate time ( from Lorentz Transformation)
In the stationary observer’s frame
- Use
as > 0 when the source and receiver are receding from each other (red shift to lower frequency [this is the current signs for ]) - Use
as < 0 when the source and receiver are approaching each other (blue shift to higher frequency)