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The Photoelectric Effect:
When electromagnetic radiation strikes certain materials,
particularly metals, electrons are ejected from them and escape
into the space around the materials. This phenomenon is known
as the photoelectric effect. Materials that behave in this
manner are said to be photo-emissive, and the emitted electrons
are referred to as photoelectrons.
The more intense the electromagnetic radiation that strikes a
photo-emissive material, the more photo-electrons are ejected
per second. A brighter beam of light, for example, causes
more photoelectrons to be emitted per second than a dimmer
one. Increasing the intensity of the radiation, however, does not result
in more energetic photoelectrons. Instead, the kinetic energy of the
emitted electrons depends on the frequency of the incident radiation
and on the type of 'photo-emissive material. The higher the
frequency, the greater the energy of the photoelectrons.
For each photo-emissive material there is a threshold frequency-
a frequency below which no photoelectrons will be emitted, no
matter how intense the radiation. For example, a material whose
threshold frequency is that of yellow light emits no electrons when,
bombarded by red light or radio waves, no matter how intense
the radiation. But this material will emit many electrons when
bombarded by even the faintest green light or by X-rays.
For every photo-emissive material, the maximum kinetic energy of
photoelectrons varies linearly with the frequency of the incident
radiation. The slope of the graph is the same for all photo-emissive
materials and is equal to Planck's constant, h = 6.6 x 10^-34 J-s).
The point at which the graph intercepts the x axis is different for
different photo-emissive materials and represents the threshold
frequency, "f" of the material. The relationship between KE and
f can also be expressed mathematically as KE = hf - Work
Function where h is Planck's constant and W, is the absolute
value of the y-intercept of the graph. This y-intercept is different
for different photo-emissive materials and is known as the
work function of the material.
If light and the other E&M radiations were just electromagnetic
waves, the kinetic energy of the ejected electrons should depend
on the intensity of the radiation, rather than the frequency, and no
threshold frequency should exist. More intense radiation (brighter
light) then means waves with greater amplitude that exert stronger
electric and magnetic forces. As a result, the electrons should emerge
with more kinetic energy when the radiation is more intense. But
since frequency, not intensity, is the sole determining factor of
the kinetic energy of emitted photo- electrons, to the point that a
"threshold" for frequency exists, a new and improved model of
light had to be devised.
The Photoelectric Effect was explained by Albert Einstein in 1905.
He proposed that light and all forms of electromagnetic radiation
consisted of particles called photons. This proposal was based on
Max Planck's quantum theory which states that electromagnetic
radiation is emitted in discrete amounts, or quanta, of energy.
Einstein extended this idea: not only was electromagnetic radiation
emitted in discrete amounts of energy, it was also absorbed in
discrete amounts, because electromagnetic radiation consists of
particles, each carrying a discrete amount of energy. This energy is
known as the photon energy or E = hf
where h is Planck's Constant.
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