Photons and electrons
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Photons and electrons
So what happens when the photons arrive? Let's look back at the case of the zinc plate at the start of this section. The photons arrive and interact with (hit !) electrons in the metal. Now here's a useful rule to follow.
Each photon only interacts with one electron.
It delivers its energy to the electron and disappears (because it is a packet of pure energy and nothing else.) So the electron now has extra energy. What can it do with it? Well, if it has enough extra energy it can leave the metal atom. Of course that means that only photons delivering enough energy will cause electrons to leave the metal.
So only photons above the threshold frequency, fo, will cause photoelectric emission.
How come more intense (brighter) radiation doesn't cause emission?
More intense radiation simply means that more packets of energy (photons) are delivered each second. But the energy of each packet is unchanged. So if there wasn't enough energy to cause photoelectric emission, making it brighter won't change anything.
Assuming we're above the threshold frequency, there are other things to think about when the photon arrives.
Photons can easily pass between or even through atoms without hitting (interacting with) an electron. So when they do finally hit an electron, there are a number of possible scenarios.
The photon hits an electron at the surface of the metal. The electron uses the energy that it has gained to leave the atom and head off to freedom. That's an easy one.
The photon hits an electron at the surface of the metal. The electron leaves the atom but heads off deeper into the metal and never manages to escape.
The photon passes deep into the metal before it hits an electron. The electron leaves the atom and heads towards the surface and escapes!
The photon passes deep into the metal before it hits an electron. The electron leaves the atom and heads towards the surface but it doesn't have enough energy to push its way past all the other atoms to get to the surface so it grinds to a halt still inside the metal and never escapes.
The photon passes deep into the metal before it hits an electron. The electron leaves the atom and heads off in the wrong direction and never escapes.
So in fact a very small proportion of the photons that arrive at the metal will cause photoelectrons to be emitted.