Need help understanding the photoelectric effect

[um4r12]

I was just trying to wrap my mind around the photoelectric effect, but I wasn't sure about why classical physics was unable to explain it. Would classical physics attempt to explain it using E=hf or amplitude. And what is it that Einstein used to explain the photoelectric effect that classical physics could not explain. Thanks, i hope the question is somewhat clear.

 

 

[John Cuthber]

Classical physics couldn't interpret it in terms of E=hf because the idea of E=hf is one of the basic ideas of quantum physics.

[Strange]

The important point is that it is the frequency of the light that causes electrons to be emitted, not the amplitude.

 

This led to the idea of the photon. A photon with a high enough frequency=energy can kick an electron out.

 

If the frequency isn't high enough, then it can't kick out an electron, no matter how many photons there are (i.e. what the amplitude of the light is).

[swansont]

The classical idea of light as a wave means that the energy of that wave is spread out over the extent of a wave. Photoionization can happen with light that has a wavelength more than a thousand times larger than an atom, so the electron should take a while to absorb the energy if this were a wave effect.

 

And, as noted above, you have E = hf, which is not classical. Classically the energy (intensity) is dependent on the amplitude squared, but a higher intensity of light does not allow for the PEE if the frequency is below the cutoff. You can have a very intense light below cutoff and a very dim light above cutoff, and the dim light, with lower total energy, is the one that causes ionization. That's not a classical result.