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Why can't photoelectric effect be explained by wave phenomenon ?

  1. Mar 3, 2013 #1
    photoelectric effect is explained by taking photon of energy 'hv'.Even electromagnetic waves carry energy.Why cant they interact with electron and transfer energy ?Even a wave has characteristics frequency and wavelength charateristics and all. I think I'm a bit confused.Can someone help me?

    PS-I'm a newbie and not a regular visitor.Sorry, if i posted in wrong section.
  2. jcsd
  3. Mar 3, 2013 #2


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    Hi Rahat34! :smile:
    But a wave doesn't have a characteristic energy. :wink:
  4. Mar 3, 2013 #3
    But when you stand in sun you feel energy being absorbed in the form of electromagnetic waves.
  5. Mar 3, 2013 #4
    that is correct. the energy of classical electromagnetic waves has to do with their amplitudes and nothing to do with their frequencies.


    this doesn't work for the photoelectric effect because below a characteristic frequency the amplitude of the wave doesn't matter, there will be no electrons scattered.
  6. Mar 3, 2013 #5
    Thanks for your reply chill factor.I understood if try to explain it with wave model then there would be nothing like frequency as energy of ejected electron would increase if intesity of light is incresed which does not happen.

    I wonder what exactly is this energy 'hv' is for a photon as frequency is something defined for a wave ?
  7. Mar 3, 2013 #6
    a quanta of electromagnetic radiation may have particle or wave properties depending on what you measure. As for what it "actually" is, it is both at the same time, which you may not have experience with in the macroscopic world, but exists all the time in the microscopic world.
  8. Mar 3, 2013 #7


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    The idea of EM energy consisting of quanta is needed to explain how, in the photoelectric effect a beam with very low mean power hitting the surface of a metal (very low average rate of energy arriving) can carry enough peak energy , in bursts, to kick individual electrons off the surface. It also explains how, with very high power radiation, of an insufficiently high frequency, electrons are not released. There is not a better alternative explanation of this.
    It does not tell us that EM energy is photons, any more than it is waves. Both interpretations are valid and different phenomena are best described using one or the other descriptions.
  9. Mar 3, 2013 #8
    A wave also has a characteristic amplitude. The amplitude of a wave is treated a little differently in classical physics than in quantum physics.

    In classical physics, the amplitude of a wave has to change in infinitesimal steps at any instant of time. Here, infinitesimal means too small to possibly detect. Also in classical physics, the rate of absorbing energy is determined precisely by the characteristics of the wave. There is no rate of transfer that is "random". Even if the atoms are exchanging energy, they can't transfer energy to one atom because all atoms are alike. The symmetry of energy transfer prevents a net concentration of energy.

    Classical physics makes a prediction that is not seen in experiment. When the electromagnetic waves enters the surface of the material, all the atoms in the materials are being exposed to the same characteristics. So each atom has to absorb energy at the exact same rate. Of course, this means that the rate of absorbing energy of each atom is extremely small. It takes a long time for the electron in the atom to accumulate enough energy to leave. So according to classical physics, each electron should be waiting "patiently" until it has enough energy to jump out of its atom. Then, the electrons should all jump out of the atom at the same time. If very low intensity light is used, the waiting time can be long enough to detect and measure. This waiting time has not been observed in experiments. Even in very dim light, some electrons jump out right way.

    In quantum physics, the amplitude has to change in finite steps which are consistent with wave-particle duality. Each "particle" associated with the wave correlates with a step in the amplitude of the wave. Furthermore, the energy transfer can have a random component in time. All atoms are alike until the random component of energy transfer makes them different.

    The experimental results are predicted by quantum physics. When light of very low intensity enters a material, all atoms are exposed to a wave with the same characteristics. The amplitude, frequency and wavelength is the same for each atom. So the average rate of energy transfer for all atoms is precisely the same as in the quantum case. However, the rate of transfer for each individual atoms is not the same as the average rate because there is a random component to energy transfer.

    The amplitude can change suddenly. In fact, the amplitude has to change suddenly. Therefore, at least some of the atoms collect enough energy to make the jump immediately after the light is turned on. The amplitude of the light wave then decreases in a finite step corresponding the the absorption of a "photon". There is a delay in jumping time which varies with each atom. For some atoms, the delay time are longer than the average time. However, the average waiting time is the same as determined by the classical theory. The experimental result that violates classical theory is that there is a finite spread in the waiting times.

    Historically, the idea of quantized amplitude came before the idea of wave-particle duality. Quantization of amplitude in a atom as a harmonic oscillator was the idea of Planck. Einstein came up with idea that the amplitude of light can come in discrete steps. However, Einstein posed the idea in terms of light being composed of particles. The two concepts are really one concept, since the physical predictions made by the theory are similar.

    Quantum physics is notoriously anti-intuitive compared to classical theory. You don't have to like it. However, it does provide predictions that are closer to experimental fact than classical physics.
  10. Mar 3, 2013 #9
    I think the "something" that you are looking for is "the square of the amplitude." In classical mechanics, the square of the amplitude is proportional to the total energy of the wave. Therefore, if the total energy of the wave is quantized in terms of "particles", then the amplitude of the wave has to be quantized in number of particles.

    Here is a link to introductory course on quantum mechanics where the idea of “quantized amplitude” is presented as fundamental concept.
    “In1905, Einstein resolved this. A key idea is quantized amplitude AN=(hN/f). (Even amplitude is wavy!)”

    The concept of quantized amplitudes has a key role in what is called “second quantization”. Second quantization replaces field variables by operators. There are some operators which have eigenvalues that are the amplitude of the waves. These eigenvalues are often quantized.

    “In terms of standing waves and superposition, a quantum wave behaves much like a classical wave. Transitions between waves of differing quantized amplitudes, of course, have a distinct quantum character.”
    “In analogy to the quantum wave, let us also introduce the novel concept of a “quantum particle” which follows a classical trajectory, but which may also have a localized coherent phase, and the trajectory may have quantized amplitude.”

    “second quantized state is uniquely characterized by the first quantized amplitude,”
    “Because vT=L , Eq. (22) does not have to be substituted into Eq. (7); therefore, the quantized amplitude is independent of n.”

    Here are more links to articles on quantum mechanics where the idea of quantized amplitude is used.
    “Thus, the Lie algebra 6.76 generates the squared quantized amplitude E2!”

    “A second quantized state is uniquely characterized by the first quantized amplitude”
  11. Mar 3, 2013 #10
    Thank you Darwin123. I'm really impressed by conceptual clarity and the helping nature of the people here. Thanks again.:)
  12. Mar 3, 2013 #11
    Rahat...since we know that the energy is distributed uniformly over some area in case of waves whereas, in case of photons this energy is confined at one spot. as i have observed.
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