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What
is the photoelectric effect?
The photoelectric
effect occurs when light hits a metallic surface and ejects
electrons. It proves that light is particulate—one of the major
foundations of quantum mechanics. In fact, explaining this theory
is what gave Einstein the Nobel Prize—not, surprisingly, for his
theory of relativity. It shows that light is made of photons, which
are particles.
There are certain electron
energy levels in an atom. If an electron absorbs enough energy,
it will jump up to the next level. If it absorbs enough energy,
it will jump up out of the highest energy level and out of the atom
altogether.
The photoelectric effect
occurs when the photon transfers enough energy to eject an electron
from an atom in a metallic surface.
In experiments, they
shone various frequencies (colors) and intensities of light at a
metallic surface. Above certain frequencies, the light would cause
the electrons in the surface to be ejected. Under those frequencies,
the photoelectric effect would not occur—the electrons remained
in the surface. They found that it didn’t matter how intense the
light was, but only that it was higher than a certain frequency.
What is the difference
between frequency and intensity? Frequency is related to the amount
of energy that the light carries. Intensity refers to how many photons
(or how much light) there is.
The higher the frequency
of light, the more energy it has. Of the visible light spectrum,
red has the lowest frequency and therefore, the lowest energy. Violet
has the highest frequency, and thus the highest energy. (Remember,
ROY G BV: red, orange, yellow, green, blue, violet.)
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How
does this prove that light is made of particles (photons)?
- There is a cutoff
frequency below which the intensity of the light doesn’t matter:
the electrons will not be ejected, regardless of how much light
is shone on the surface.
Classical physics, which
assumes light is only a wave, predicts that the frequency (energy)
of the light wouldn’t matter, given enough intensity. In other words,
as long as there is enough light, Newtonian physics predicts that
the photoelectric effect should take place.
This, however, is not
the case. Experimentally, below the cutoff frequency for a metal,
the photoelectric effect will not take place, regardless of how
intense the light is.
Light acts particulate
in this situation. The energy is transmitted in little packets,
or photons. When a photon hits an atom, it only carries a certain
amount of energy with it. Electrons must absorb more than certain
amount of energy to be ejected from the atom. So if the photon doesn’t
have high enough a frequency, and thus high enough energy, the electron
won’t be ejected from the atom. It doesn’t matter how many photons
hit the atoms: the energy is not cumulative.
In an imperfect analogy,
imagine a steep ramp (like the energy levels in an atom), and a
ball at the bottom (the electron). Give the ball a gentle push (like
the energy an electron absorbs from a photon), and it rolls a certain
distance up the ramp before returning. If you give it a large enough
push, the ball will roll all the way to the top of the ramp and
off the edge (like when an electron is ejected from the atom). It
doesn’t matter how many gentle pushes you give the ball. If you
don’t give a large enough a push, the ball will never reach the
top of the ramp and fall off.
(But this isn’t a totally
accurate analogy. The energy levels of an atom are quantized.
They are more like the steps in a staircase, because electrons are
confined to certain orbitals and cannot go anywhere they want to.)
- When at the same frequency
(above the cutoff frequency), the electrons will be ejected with
the same kinetic energy (speed), no matter how much light is shone
on the surface.
If light acted as a wave
in this situation, the higher the intensity, the higher the energy
transferred to the electron, and the higher the resulting kinetic
energy of the electron. Basically, classical physics says that the
more light, the more energy, the faster the electron would travel
from the atom.
But light acts particulate.
All photons of the same frequency have the same amount of energy.
Each photon gets one chance to affect one electron. It carries with
it a certain amount of energy which it then transfers to the electron,
giving it speed. The intensity of the light doesn’t matter. Regardless
of how many photons you shoot at the surface, each photon affects
only one electron. So all the electrons that are ejected get the
same amount of energy, and therefore have the same amount of kinetic
energy/speed.
The experiments confirm
this. When exposed to light of the same frequency, electrons are
ejected at the same speed, regardless of intensity.
- When the frequency
(and thus energy) of the light is increased, the kinetic energy
of the electrons also increases.
That’s pretty self-explanatory.
Increasing the intensity of the light will not increase the
kinetic energy of each ejected electron. But increasing the frequency
of the light will give increase the kinetic energy of each electron.
If I send a two photons instead of one, that won’t affect the speed
at which the electron is ejected. But if I increase the amount
of energy that one photon carries, that will indeed increase
the speed at which the electron is ejected.
- The electrons that
are emitted from a surface do so almost immediately, even at low
intensities.
If light acted like a
wave in this situation, we would expect a delay as the electron
absorbs more energy from the low intensity light.
On the contrary, experiments
show that even just a few photons will almost instantaneously eject
electrons from the surface.
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Light
is particulate...but it can also act like a wave!
We’ve spent all this
time proving to you that light is particulate. It is. But it has
been experimentally shown (see interference)
that it also behaves like a wave…
In fact, all particles
(including the atoms that make up you and I) sometimes act like
particles and sometimes act like waves. But the thing is that they
are both particles and waves and exhibit the both properties at
different times. Newtonian physicists were partially correct in
thinking that light acts like a wave. But quantum mechanics
For more information,
see the wave-particle duality
of light.
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1.
Serway, R. A.
2. Thomas, D.
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