Question

Explain this please: "Sodium is irradiated for 2 seconds with infrared light and no electrons are ejected. The same metal is irradiated for 2 seconds with ultraviolet light and thousands of electrons are emitted."

Answer 1

Since ultraviolet light is of higher wavelength (and thus, energy) it is able to excite the electrons in the sodium to a greater degree.

Answer 2

@Benjammin the UV light has a higher frequency, not wavelength, and therefore has higher energy.

Answer 3

You are totally right, sorry! Should have stopped to proofread myself.

Answer 4

The thing that makes this experimental observation curious -- and therefore the reason you're asked to study it -- is that both IR and UV light supply gobs of energy, more than enough to eject electrons from sodium metal. To be sure, the IR light supplies less energy in a given time than the UV, but that just means you should get fewer electrons per unit time (a lower photocurrent).

Yet you get not fewer, but absolutely zero when you use the IR, no matter how long you shine it on the metal. Clearly there is something strange going on. Imagine a classical example: you have some satellites going around the Earth, and you (an alien invader) start shooting at them. If you shoot low-velocity bullets, then you would certainly expect to make fewer satellites come out of their orbits (either to crash on the Earth or be ejected from orbit entirely). But you'd naturally expect that the impact of many of the low-velocity bullets could easily add up to the same effect as the impact of a few or just one high-velocity bullet. So the lower the velocity of your bullets, the fewer satellites you expect to crash, but you expect the number to go down smoothly and steadily with the bullet velocity. Imagine instead that what you see is that you get satellites crashing above a certain bullet velocity, but when you reduce the bullet velocity even a hair below that key velocity, you get suddenly none at all. You would begin to suspect there is more going on than just the energy your bullets supply.

So it is with the photoelectric effect. The strange thing is that the electrons will not absorb photons with smaller energies at all, so an electron cannot build up with the absorption of a number of photons the energy required to escape the metal. If an electron cannot absorb enough energy to escape the metal from a SINGLE photon, in one shot, it won't be able to absorb it at all.

This is very peculiar! And this is one of the key observations that leads to quantum mechanics. Because it says in certain situations, the energy of a particle (in this case an electron bound in a metal) cannot change in small increments. It must change in sudden jumps -- quanta -- if it is to change at all.

Answer 5

/me high-fives @Carl_Pham