Newton described light according to his mechanistic
ideas as particles (Quanta).
Christiaan
Huygens however in 17th century introduced elementary waves to describe
light features. The wave nature of light has been introducing by innovative
diffraction experiments using double slit by Thomas Young
in
1801. And after that nobody doubts the wave nature of light any more because all
phenomena can be described in a correct way after this theory. The Plancks
Black Body Theory was quite pioneer theory which describes correctly only this
phenomenon but generally it gives (by introduction of quanta) more problems than
solutions. However, there were held more experiments results of which could be
described only applying quanta. To put it more exactly, light behavior became
more complicated to understand since sometimes it can reveal particle features
and sometimes it can reveal wave characteristics. However, we would like to turn
to few experiments which distinctly prove the particle features of light: the
photoeffect and
the Compton effect:
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If electromagnetic radiation having quite high frequency falls onto the metal
surface electrons will be emitted which are also referred to as
photoelectrons. This is the photoeffect.
Classical description:
The electromagnetic field E
of the incident light wave produces the vibrating force acting on free electrons
in metal. It follows that electrons are not emitted when applying light with
higher amplitude (not frequency). It is conflicted with experiment.
Experiment:
There are no photoelectrons below frequency νo
.
The electron velocity doesn't depend on the light power.
The photoelectrons are also immediately emitted when applying weak light
power.
Quantum description:
The energy quant hν is absorbed. Since electron
is bounded in metal it needs to have part of this energy in order to do the
photoelectric work WA. The rest of energy will be free as the kinetic
energy.
½ me v2 = hn- WA |
Analog with Football:
In football one wants to kick ball with mass m over the hill H.
The kick gives a ball its initial energy E which is similar to that for
photoeffect. If ball overpasses this small hill then it loses potential energy mgH
that is similar to photoeffect work WA. That's why the ball velocity
v on the hill will be as follows:
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½ m v2 = E − mgH |
The Compton effect is the most conclusive evidence of a particle
electromagnetic behavior.
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Experiment:
The Roentgen beams fall on graphite and then diffract in all directions. The
diffracted radiation has component with the same frequency as the incident
emission whereas other component reveals lower frequency. The frequency is
higher the bigger angle θ is.
Energy conservation:
Energy before collision = Energy after collision :
hν + m0c2 = hν’ + mc2
hn − hν’ = mc2 − m0c2
ν -n' = m0c²/h{[1/(1 − v²/c²)½]− 1}
where we used the Einstein expression mc2 = moc2/(1
-n2/c2)½ .
Photon gives its energy to electron and therefore its frequency becomes
lower (ν’ <n).
Impulse conservation:
Photon has an impulse since after Einstein:
E = mc2 = p · c = hν = hc/λ
p = h/l p = hν/c |
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Impulse p = h/cν' | ; px = h/cν' · cosθ |
Impulse p = m · v | ; px = m · v · cosj |
1/ν' = 1/ν + h/m0c²(1 − cosθ)
λ' = λ + h/moc (1 - cosθ) |
The quantity h/moc is the Compton wavelength and it is equal to 0,00243 nm.
The maximum wavelength change can be detected for θ = 180° (backwards diffraction).
Dlmax = λ' - l = 0,00486 nm (it doesn't depend on the wavelength)
Classically there is no any wavelength change, since h = 0 ! |
The predictions are proved by an experiment:
Summing up all these experiments one can say: Photon behaves itself like a particle !