If a train is moving with its backlight on, a man standing on the track behind the train will see it red shifted. Now let us suppose that the light was initially blue and its red shifted enough for the man to appear almost red. Now if we keep a photoelectric plate at where the man was standing, that emits electrons under blue light, will electron still be emitted?
3
-
$\begingroup$ Anyone answering it, please consider this question too: physics.stackexchange.com/q/869761 $\endgroup$gm1210– gm12102026-03-04 07:49:52 +00:00Commented 21 hours ago
-
1$\begingroup$ No, photoelectrons will not be emitted. Can you say why you think they might be emitted? $\endgroup$John Rennie– John Rennie2026-03-04 09:17:26 +00:00Commented 19 hours ago
-
1$\begingroup$ @John Rennie I was confused with whether the actual properties of the light wave were changing or it was just the observer's perspective. $\endgroup$gm1210– gm12102026-03-04 12:08:42 +00:00Commented 17 hours ago
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
1
Yes, the photoelectric plate held by the man will stop emitting electrons once the light has red-shifted sufficiently enough and the photon energy observed by the man/plate $h \nu_{\rm observed}$ becomes less than the work function of the material.
There is no contradiction here: even in the train's frame of reference, the train driver wouldn't observe photoelectric effect taking place because the photoelectric plate is now moving in their reference frame and they understand that to determine whether photoelectric effect takes place or not, one must find the frequency of light hitting the photoelectric plate in the plate's rest frame.
In OP's other question, they asked
So basically, is the light actually changing or its just for the observer?
The frequency of light emitted by a source is actually different in different frames of reference. The red-shift is not some kind of optical illusion that is only perceived by the man's eyes; the photoelectric plate does detect the red-shift.
-
$\begingroup$ "one must find the frequency of light hitting the photoelectric plate in the plate's rest frame." The physics should be describable in either frame. In the train's frame, what is it about the plate moving away that makes it unable to eject electrons when the photons reach it? I'm guessing it's because the relativistic motion of the plate changes the energies involved in the electron orbitals? I've had this same question before and wondered if there are more details known about how material properties change at relativistic speeds $\endgroup$HiddenBabel– HiddenBabel2026-03-05 00:30:39 +00:00Commented 4 hours ago