Photon Frequency and Energy
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?
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
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.
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.
Click here to return to the Physics Archives
Update: June 2012