Quanta

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:

The Photoeffect

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:

½ m v2 = E − mgH

½ mv2 = E − mgH
gegeben, was analog zu
½ mev2 = hn- WA
One can find the photoeffect application for instance in photomultipliers. The avalanche amplification of emitted photoelectrons is used even for the detection of single photons (photon counting regime). It's detected one photon of about 3 photons on average. The human eye is also quite sensitive after a long accomodation period: some 10 photons pro second are enough to see a light.

Light as Billiards Ball: The Compton Effect

The Compton effect is the most conclusive evidence of a particle electromagnetic behavior.

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' = m 0c²/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

| Impulse p = h/cν' | ; px = h/cν' · cosθ
Impulse p = m · v | ; px = m · v · cosj

1/ν' = 1/ν + h/m 0 c²(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 !