Why do electrons in the photoelectric effect move towards the direction of where the photons came?

Question

Why do electrons in the photoelectric effect move towards the direction of where the photons came? Intuitively I would have said that the electrons would go in the same direction the photons go. But the experiment shows they don't, they go towards the direction the photons came from. Why is that?

Answer 1

You're thinking that the momentum of the photon should drive the process. But an optical or UV photon has much less momentum than an electron of the same energy, so the photoelectric absorption needs another source of momentum.

The material provides it. A bound electron is constantly exchanging momentum with the nuclei binding it. Basically, when it interacts with a photon, it keeps the momentum it has.

We see the importance of this momentum transfer in UV and x-ray photoelectric cross sections, where inner shell electrons with more momentum are much more likely to absorb photons if the energy balance works.

Classically, you'd expect a different result: the electric field in electromagnetic radiation is transverse, so you'd expect the electron to go sideways. We see a bit of that in silicon exposed to polarized x-rays: the resulting charge clouds due to the ionization by photoelectrons tend to be stretched in the polarization direction.

Answer 2

The direction of the photoelectrons(emitted electrons) is opposite to the incident photons because of the conservation of momentum. Let's consider a hypothetical example to understand this: Imagine a photon travelling along the x-axis and strikes an electron(placed on the x-axis). Now, in the photoelectric effect, the photon is absorbed by the electron. Due to the complete disappearance of the photon, the electron will gain momentum in the opposite direction(-x axis) to conserve the momentum.