  We present a simple thought experiment in which a beam of light of a finite
length (a light-pulse) enters a detection device that consists of a wave
detector and a light bulb. We examine the experiment from the reference frame
where the device is stationary and from the reference frame where it moves at a
constant velocity along the axis of the beam. We use basic special relativity
to derive the formulas that show how the length of the light beam is
transformed between the reference frames. We further show that the uncommon
formulas for length transformation of a light beam do not contradict the usual
relativistic result that a uniformly moving non-light object will be Lorentz
contracted along the direction of its motion.
