Contents
The nucleus
Radioactivity
Radiation Measurements
Black body radiation
Statistical Mechanics
Radiation and scattering
Related topics
Related links
© The scientific sentence. 2010
|
Phoelectric effect
After Planck's formula on black-body radiation, the quantum hν
received many applications. Albert Einstein, in 1905, Postulated that
light, electromagnetic wave, exist in corpuscles form. Each
corpuscle holds an enrgy quantum hν; ν is the frequency of the
related electromagnetic wave. It followded that light is quantized.
This postulate was necessary to explain the phenomenun of the extraction
of electrons from a metal, which is called the photoelectric effect.
When light shines on a metal, the light particle can extract an electron
from this metal only it has a enough energy to extract it, no matter the
number of light particles, that is the intensity of the light. The
important here is that the energy hν of the quantum light
particle is at least equal to the work of extarction "W" of an electron.
The frequency minimum νo is called the threshold frequency.
Then the extration of an electron is possible if hνo >= W.
The frequency νo is just the needed value for extraction; if the
frequency ν is greater that νo, the surplus energy is a
kinetic energy for the electron that serves to move away the extracted electron.
We can write this as:
hν = hνo + mv2/2
νo = W is the extraction work,
m mass of an electron and
v the electron gained velocity.
In 1887, Hetz had experimented the negative charge (electrons) extraction
from a metallic plate by ultraviolet rays. The number of the extracted
electrons was proportional to the uv intensity, but their velocity was not
dependent to this intensity. This veloccity was dependent only on the frequency
(or the wavelength) of the used uv shining the metal. Plus, there was a maximun
wavelength below which no electron was extracted. we can think that Hertz
already experimented the photoelectric effect formula.
Robert Millikan, to check the validty of the photoelectric formula, had measured
the voltage between a plate (receptor) shined by light, and a cell (collector)
with respect to the frequencies. He had found the ration h/e and the value of
h at 1% of the Planck's value.
In 1923, The Compton effect confirmed tha validity of the photoelectric
effect. In this effect, the collision of the light particle on the electron,
changes the direction and the frequency of the incident light particle; while
the electron target recoils. In this year 1923, the light particle received
the name of photon.
|
