franklinhu@yahoo.com wrote:

Now we all know that light has been proven to consist of particles

to the photoelectric effect, but do we really?

Not "proven to be". With some assumptions (e.g. "light is only
absorbed by atoms in discrete quantities, but is continuous in free
flight), one could still explain this with waves. But the point is
that such assumptions would be totally farfetched and unjustified,
themselves in need of an explanation, and further are contradicted by
other evidence like e.g. the Compton effect. In other words, the

effect (and other effects) provided *strong* *evidence* for light
consisting of particles - but not proof.

Based on descriptions
found on websites and books such as "The Evolution of Physics" by
Albert Einstein/Leopold Infeld, not much thought has been put into

waves ought to do when shined on a metal surface. It is surmised
(without any further reasoning) that the intensity of the light

would correspond to its amplitude or energy level,

Please define what exactly you mean with "intensity" and "energy
level" here.

so we would expect
that higher intensity would increase the speed of the electrons

the surface. We don't see this experimentally, so we must conclude

light is a particle.

That's only part of the reasoning. You conveniently ignore the other
part: that the speed of the electrons indeed increases when the
frequency is increased.

However, based upon what we know about how light is generated,

not a foregone conclusion. Light is generated by an electron
transistioning between energy levels within the atom.

That's one of several possibilities only.

As such, the
duration of the wave generated is limited - probably to only a

wave

"single wave" makes little sense. Do you mean "single period"? If

you are utterly wrong. Light frequencies are around 10^15 Hz, light
emission takes about 10^(-10) s. Hence there are 10^5 periods in

wave train emitted.

and its amplitude is also limited since the range of motion from
energy level to energy level is limited.

The amplitude of the emitted wave has precisely nil to do with
the "range of motion from energy level to energy level".

This kind of wave would be
like what you would get if you took a taught rope and jerked it

very quickly.

Wrong, false analogy and false numbers.

The single wave would travel down the rope.

Again, "single wave" makes no sense here.

A beam of
light would be a series of these small waves travelling through

No, it isn't. See the numbers above.

And even if it were: why couldn't several of these small wave trains
be at the same place and add up?

It would not be a continuous wave of variable amplitude. The

would not be due to the amplitude of the individual waves, but

the density of waves hitting an object.

I can't comment on that as long as you don't tell me how exactly
you define "intensity".

A bright object is simply
emitting more of these "single" waves per unit time than a dim

Care to tell me what the big difference (conceptually) between your
concept of "single waves" and the concept of photons is? You *do*
know that photons are often described as *wave packets*, don't you?

[snip more of that]

This seems terribly obvious based upon what we know about light
generation, so why is this argument not brought up?

Essentially, it is. Photon = wave packet is an idea which has
been around for 70-80 years now. It works for many applications
- but ultimately, it fails if you look closer at the idea.

Bye,
Bjoern

franklinhu@yahoo.com wrote:

Now we all know that light has been proven to consist of particles due
to the photoelectric effect, but do we really? Based on descriptions
found on websites and books such as "The Evolution of Physics" by
Albert Einstein/Leopold Infeld, not much thought has been put into what
waves ought to do when shined on a metal surface. It is surmised
(without any further reasoning) that the intensity of the light wave
would correspond to its amplitude or energy level, so we would expect
that higher intensity would increase the speed of the electrons from
the surface. We don't see this experimentally, so we must conclude that
light is a particle.

However, based upon what we know about how light is generated, this is
not a foregone conclusion. Light is generated by an electron
transistioning between energy levels within the atom. As such, the
duration of the wave generated is limited - probably to only a single
wave and its amplitude is also limited since the range of motion from
energy level to energy level is limited. This kind of wave would be
like what you would get if you took a taught rope and jerked it once
very quickly. The single wave would travel down the rope. A beam of
light would be a series of these small waves travelling through space.
It would not be a continuous wave of variable amplitude. The intensity
would not be due to the amplitude of the individual waves, but rather
the density of waves hitting an object. A bright object is simply
emitting more of these "single" waves per unit time than a dim object.

Now think of how such a wave would interact with the surface of metal.
Since the amplitude of the wave doesn't change, electrons are ejected
from the surface with the same energy no matter how many hit the
surface. The speed of the ejected electron would depend on the amount
of kinetic energy imparted by the wave which would correspond to the
frequency of the wave as is demonstrated by the fact that you can shake
out nuts with greater speed out of a tree if you shake it faster. This
is exactly the behavior seen in the photoelectric effect without
resorting to thinking of light as particles.

This seems terribly obvious based upon what we know about light
generation, so why is this argument not brought up?

fhuphoto

Your argument, taken in context, is reasonable. My version involved
window panes. A series of sonic waves impinging on a wall of identical
window panes would result in some breaking and others not, even though
all identical. Now add to this resonance effect a preexisting
oscillation of the panes (analogous to thermal activity) and in this
case only a single incident wave will cause some panes to shatter and
some not, all still identical but differing in phase superposition with
the incident wave.

IOW, Carlip's argument that the energy must *build up* is not even
wrong. The electrons, like the agitated window panes, already have
energy, and some are already ejecting from the surface even without
incident radiation. Thus one can expect that an addition of energy from
an outside source would increase the probability of ejection. It isn't
that simple, but it isn't so very complicated either. Thermions are
evidence enough of the general idea.

Photoelectrons, OTOH, are linked to radiant energy above a given
frequency. In order to explain this relationship lets go back to the
wall of window panes. Note first that they have a natural resonant
frequency, and thus we can expect that impinging waves of this frequency
will lead to a drastic change in the probability of pane breakage over
that incurred from lesser frequencies, the latter of which will tend to
flex all of the windows simultaneously, thus increasing pressure inside
the building, which in turn counters the inward flexing of the windows,
i.e. these lower frequency waves are dampened in the material. The wall
of windows will act essentially as a low pass filter. Higher frequencies
will be absorbed because a single pane cannot oscillate at these
frequencies in its entirety; it's dimensions are greater than the
wavelength, and thus local oscillations cause counter forces on the pane
as a whole.

Internal pressure fluctuations in the structure are
dampened as well.

The electronic version is that internal electromagnetic "pressures"
superpose over the radiation pressures, canceling local effects on a
given electron, on average. Higher frequency incident radiation isn't
dampened locally, because the interference nodes are local, i.e.
affecting single electrons differently than their neighbors.

Richard Perry

franklinhu@yahoo.com wrote:

Now we all know that light has been proven to consist of particles due
to the photoelectric effect, but do we really? Based on descriptions
found on websites and books such as "The Evolution of Physics" by
Albert Einstein/Leopold Infeld, not much thought has been put into what
waves ought to do when shined on a metal surface. It is surmised
(without any further reasoning) that the intensity of the light wave
would correspond to its amplitude or energy level, so we would expect
that higher intensity would increase the speed of the electrons from
the surface. We don't see this experimentally, so we must conclude that
light is a particle.

There's more to it than that, though the conclusive experiments are
relatively recent. The basic reason to conclude from the photoelectric
effect that light is quantized comes from the appearance of "prompt
electrons," electrons that are kicked out of the surface before a wave
could have built up enough energy to eject them. Unless you're ready
to give up energy conservation, the observation of such prompt electrons
is pretty conclusive evidence against a wave picture.

The best experiment I know of is described in Davis and Mandel, in the
book _Coherence and Quantum Optics_ (ed. Mandel and Wolf, 1973). This
is a very careful observation of prompt electrons, emitted before there
would be time, in a wave model, for enough energy to build up even over
the entire cathode to overcome the potential barrier. The experiment
shows that the energy is *not* delivered continuously to a photodetector.
They do more, as well -- they look at the statistics of prompt electron
emission, and show that it matches the quantum mechanical predictions
for the distribution of photons.

Steve Carlip