1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Use Max Planck's quantum theory to explain the following..

  1. Mar 15, 2017 #1
    1. The problem statement, all variables and given/known data

    Use Max Planck's quantum theory to explain the following behavior of photoelectrons.

    i) Low-intensity light does not release any photoelectrons. What will happen if the light is made brighter? Explain your reasoning.

    ii) Low-intensity light releases photoelectrons. What will happen if the light is made brighter? Explain your reasoning.

    iii) Low-intensity light does not release any photoelectrons. What will happen if the frequency of the light is gradually increased? Explain your reasoning.

    2. Relevant equations

    3. The attempt at a solution


    If low-intensity light does not release any photoelectrons and light is made brighter, nothing noteworthy would happen. The reason for this is because the intensity, or brightness, is only a quantity of the rate at which the photons strike the surface. In addition, providing extra photons per second does not aid in releasing extra electrons, since each single photon does not have adequate energy to do this on its own. Also, metal can emit photoelectrons as long as the frequency of light is above a specific frequency, which is known as threshold frequency. As a result, since no photoelectrons are released, nothing significant will occur since the frequency is below the threshold frequency.


    If low-intensity light releases photoelectrons and light is made brighter, frequency is above the threshold frequency which means each photon will have sufficient energy to generate photoelectrons. Additionally, increasing the intensity or brightness, will increase the number of photons, which will increase the number of photoelectrons. Conversely, the maximum kinetic energy of each photoelectron will not be affected by the larger number of photons. As a result, increasing the brightness of the light will increase current in the circuit.


    If low-intensity light does not release any photoelectrons, it must mean that the frequency of the light is under the threshold frequency. As frequency progressively increases, the kinetic energy increases. It will eventually go over the threshold frequency which is the minimum frequency to release photoelectrons and will then start releasing photoelectrons. Subsequently continuing to increase the frequency over the threshold would not increase the current and the current will be the same.

    I was wondering if someone could review my answers. I feel I might be missing something as this portion of the unit was challenging for me to understand.
  2. jcsd
  3. Mar 15, 2017 #2
    Your answers are correct. I see only one significant correction:
    When we talk about "kinetic energy" in this context, we are (I think) referring to the energy of electrons after they get emitted. Below the threshold frequency there is no kinetic energy to speak of. Better to write "the photon energy".

    It is possible that your teacher chose to refer to the photons' energy as "kinetic energy", in which case what you wrote is correct. Classifying energy as being "kinetic" or not can be arbitrary sometimes.

    Other than that, you just need to "clean up" your answers a bit. They are kind of long. Some sentences can simply be erased.

    One interesting point: in the case where the intensity is held constant and the frequency is increased (above the threshold), it seems to me that the current will decrease. Can you think of why? Don't worry, I don't think this point is what the teacher was looking for.

    BTW, the "photon" explanation for the photoelectric effect was given by Einstein, not Planck. Planck came up with the photon idea to solve a different problem, the blackbody radiation spectrum. Einstein received a Nobel Prize for this, but never for Relativity!
    Last edited: Mar 15, 2017
  4. Mar 15, 2017 #3

    ^__^ Thank you extremely much! I was struggle in this question because I was confuse myself with other concepts. But now I'm much more confident in it.

    Yes, I will definitely review my sentences (English is my second language so I often make it as simple as possible before tiding it up.)

    To answer your question (Based on what I know.) would it be because wavelength is increased? As wavelength is increased, current decreases?

    Again thank you for looking for my work!
  5. Mar 15, 2017 #4
    No, increasing frequency actually implies decreasing wavelength. The wavelength times the frequency always equals the speed of light.

    As for why the current will decrease- think about the number of photons...

    Oh, your English is actually quite good! I meant that you should clean up by making your answers more concise. There is some repetition & some irrelevant information.
  6. Mar 15, 2017 #5
    Once we're on the topic, check out this Physics Forums Insights article:
    It turns out that the fact that light is composed of photons is actually not necessary in explaining the photoelectric effect! Most people, even physics teachers, don't know this. It even surprised my Quantum Physics teacher, who is a professor of high energy physics!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted