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Name: Jonathan
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Question:
In the photoelectric effect plank states that E=hf. this means that the energy of the photon is proportional to the frequency. (This proves the particle theory of waves as increasing amplitude/brightness which represents and increase in energy has no effect on removing an electron). So why does increasing the frequency of the wave result in the photons having more energy?



Replies:
Jonathan,

When a material has greater energy within it, the individual molecules within the material vibrate at a greater frequency. When electrons within an atom have greater energy, they orbit more quickly. To generate a higher frequency light wave, the reaction that produces the wave is a reaction that gives up greater energy per photon.

Of course, a common question is "What is a photon?". A photon is a "particle" of light. A beam of light is a great many photons moving together. If you increase the intensity of a beam of light without changing the frequency, you actually increase the number of photons. When an electron drops from an excited state to its ground state, the electron releases one photon. The concepts of "intensity" and "amplitude" do not apply to an individual photon. For an electron, energy can show up as a combination of speed and mass. For a photon, mass is zero and speed is always the same. Frequency is perhaps the only variable available to act as the indicator of the photon's energy.

I do not know why the universe is "designed" to have photons behave as such. Fortunately, it does work. Simpler systems could work on large scales but would fall apart at the level of individual particles. Without the quantum effects that do not allow a photon to lose part of its energy but maintain its frequency, there would be nothing to hold photons together as they travel through empty space.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


We can come up with reasons, but you are getting to fairly fundamental questions, where the best answer may just be "because that is how it is." Much of quantum mechanics, the physics of all this, starts with this E=hf type thing, and builds up. It is related to the uncertainly principle, which also is often taken as just "the way things are." We use it because it works.

Steve Ross


First, for proper attribution, it was Einstein, not Planck, who explained the photoelectric effect, even though the constant "h" is called Planck's constant. The experimental observations are as follows:

If a metal target, charged with respect to another electrode is arranged so that the energy of the electrons emitted from the metal's surface and the number of electrons emitted can both be measured, and the metal is irradiated with "light" of a certain frequency "f", no current due to electrons emitted from the metal target is observed until a critical frequency is reached -- no matter how bright the intensity of the irradiating "light" (the word "light" is in quotation marks because the electromagnetic radiation is not necessarily in that part of the spectrum that is visible to the human eye). This critical onset is called the "work function" for historical reasons.

When a critical frequency of irradiating light is reached, electrons of a specific kinetic energy are emitted from the metal. This energy, which can be measured by an appropriate experimental

The important observation is that the energy of the emitted electron does not depend upon the intensity of the "light". The current, that is the number of electrons, depends upon the intensity of the impinging radiation, but NOT the energy of those electrons.

In contrast, the electric current, that is the number of electrons emitted from the metal's surface, IS proportional to the intensity of the impinging "light".

Do not confuse the energy of the electrons emitted with their number, that is their intensity. There can be a lot of electrons (large electron current) all with a small energy, or there can be a few electrons (small electron current) each with a large energy.

This result: Electron energy proportional to the frequency of the irradiating "light" above a certain critical value, but with the number of emitted electrons (i.e. electron current) proportional to the intensity of irradiating "light" is totally inconsistent with the classical wave model for electromagnetic radiation, which requires that the energy of the impinging "light" is proportional to the square of the amplitude of the impinging "light" (that is -- its intensity).

I think that you are confusing the energy of the emitted electrons and the number of electrons being emitted.

Vince Calder



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