Whats the main features of the photoeletric effect that cannot be explained by classical physics? ?

Question

any help would be greatly appreciated

Answer 1

to answer your question, i should remind that in classical physics the energy of an electromagnetic wave is proportional to its squared amplitude... but by photoelectric experiment it is revealed that the energy of an electromagnetic wave is mainly related to its frequency, furthermore only some discrete frequencies are able to provide the needed energy for electrons to release, so the energy levels of atoms are quantized in the contrary to classical physics that considered energy as a continiuous quantity...!

Answer 2

Classical Physics couldn't explain....

1.  Shining low freq/long wavelength light (eg RED) onto a metal doesn't produce a photoelectron (that is an electron that has been ejected from the metal because it 'absorbed' too much energy from the light) no matter what the Intensity, or brightness of that RED light.

Classical Physics says that light is a wave, and if you increase the Intensity or brightness (which is the Amplitude of the wave) you would increase the energy of that light.  E ~ 1/2 (amplitude)^2

So if you increase the brightness of red light, sooner or later, that light should have enough energy to eject an electron from that metal.

BUT this was not seen in the photoelectric effect.

Furthermore If you shone medium frequency/medium wavelengh light (say Yellow)... even at very very very DIM levels, you got Photoelectrons flying out of the metal.

If calculations were made.. say of the E(nergy) of Bright Red light, versus the E of the very very Dim yellow light, the E of the yellow light was MUCH less than the E of the Bright Red light.  But still the yellow light was able to eject an electron from the metal!!!

2.  For the yellow light, if you increase the intensity of the light, Classical theory predicts that you would get more electrons with more energy. However when you increased the intensity of the yellow light, you only got more electrons coming out of the metal,  but electrons did not have more energy.  ie there was more current, but the stopping potential did not increase (or the electrons did not have more kinetic energy flying out of the metal.. )

3.  As you increased the frequency of light, say to Blue light, you also got photoelectrons, but this time.. the electrons had a lot of kinetic energy, even with dim intensity, these electrons had more kinetic energy than the electrons ejected by the high intensity yellow light.

Hence, using the classical theory of Light (as a wave) can't explain the PE effect.  Einstein/Planck then changed the world by proposing that light is a particle, called Photon to explain the PE effect.