If you consider a blinking light source that is receding from an observer, what happens to its wavelength?

Question

decreases

increases

needs more information

remains the same

Answer 1

For light in a vacuum, there is no "medium" the velocity of which has to be considered.

Instead, the frequency that results, f', has the following relationship to the frequency f from the source:

f' = (f) (1 - u/c) / sqrt[1 - (u/c)^2]

u = speed of separation of source and observer

c = speed of light

(use +u if they are approaching each other).

The blinking frequency, presumably much smaller than the light's oscillation frequency, would seem to follow the same pattern, but there may be subtleties there having to do with the apparent time in one inertial coordinate system versus the other.

This causes an apparent "redshift" for seating objects and a "blueshift" for those

approaching one another, and can be used by astronomers to measure relative motion.

Note that u/c = 0.1, 10% of the speed of light, produces

f'/f = (0.9) / sqrt[1 - 0.01] = 0.905

about a 10% reduction in the apparent frequency, a redshift.