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Photoelectric Effect and Photons as Particles
Name: Timothy
Status: student
Age: N/A
Location: N/A
Country: Great Britain
Date: N/A
Question:
Why does the photoelectric effect prove photon
particles?
I was reading about the photoelectric effect of light, and lights
duality. It appeared that most everything can be explained by the
wave aspect except for this effect. Anyway something came to mind I
do not understand. Why would the photoelectric effect prove the
particle aspect of light? I know that the frequency is key to effect
and not the amplitude, but this seems more like a wave effect then a
particle effect. I know when I ride my boat I like the big rolling
waves. My boat rides over them with easy because the up and down
movement is not so sudden, the height of the wave does not seem to
matter as long the wave is long enough. In fact I sometimes do not
notice them at all. However when the water is choppy meaning there
is a whole bunch of small amplitude waves at high frequency hitting
the boat, I come home with a sore butt, and everything in the boat
get tossed around all kinds of ways, do to constant up and down
movement. So it would seem that amplitude is not always as
destructive as the frequency.
So if it is waves that knock me around why does it have to be
photons that knock atoms around, why not just the energy in light
waves?
Replies:
First, I like how you think. Thinking about yourself in the place of a
particle (or any other subject of study) is a great way to analyze
scientific subjects.
Unfortunately, quantum effects tend to defy traditional notions of 'how
things work'. In this example, light waves differ from water waves in many
important ways. In the photoelectric effect, light hits some material which
absorbs it, and then ejects incident electrons. The reason that the
photoelectric effect is evidence for the particle nature of light has to do
with how materials absorb that light energy and then eject it in the form of
electrons.
As you have read, scientists believe that light consists of individual
packets called photons, with discrete amounts of energy (they are also
called 'quantized', or made of quanta). For now, I'd like you to forget
that. For the sake of argument, suppose light is purely waves, not photons.
If light were not quantized, you could create light of any energy intensity
you wanted regardless of the frequency. For instance, you could take some
light of one frequency and adjust its intensity such that it equals the
intensity of light of a different frequency. If this were true, using the
photoelectric effect (where light is converted to electrical current), you
could change the energy of incident electrons purely by changing the
intensity of light, regardless of its wavelength.
It turns out, experiments do not support this. You cannot affect the energy
of incident electrons by changing the intensity of light. You can eject more
of them, but they turn out to all have the same energy. In contrast, the
frequency is very important in the photoelectric effect. The only way to
increase the energy of the electrons ejected in the photoelectric effect is
to adjust the light's frequency. This shows that there is something about
the nature of the light, not just how much of it there is, and suggests that
light cannot be infinitely divisible as pure waves would be.
So it turns out that photovoltaics, like much of quantum physics, are not
very much like your boat or you butt. :)
Hope this helps,
Burr Zimmerman
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Update: June 2012
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