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Photoelectric Effect
What it is, who came up with it, what is has to do with quanta, and who did an experiment with it
12th Grade

Additional Physics Flashcards




Photoelectric Effect (make a picture showing two situations. One with electron being liberated and one with photon being absorbed

The photoelectric effect is the emission of electrons when a metal is illuminated or "hit" by light.


It is a one-for-one trade off. One photon for one electron.


Electrons that are emitted via photoelectric effect are called photoelectrons


In order for an electron to be released (photoemission to occur) the light hitting the metal must have a minimum frequency.


This minimum frequency is called the "threshold frequency".


If the incident light's frequency is BELOW the threshold frequency required by the metal, an electron will NOT be released. Rather, the incident photon will be absorbed.

Einstein's Contribution
Einstein applied Planck's idea of EMR being quantized to light. He said light energy hits the metal surface in discrete bundles (photons).

Einstein then said the incident photons must have a minimum amount of energy in order for an electron to be released.

Work function: the minimum energy that a photon can have to cause photoemission from a metal. It is different for every metal.

W = (h)(f0)

Since work function and threshold frequency are related, if an incident light frequency is BIGGER than the threshold frequency, the incident light's photons have enough energy for photoemission. (meet f0 = meet W)

Work function(W) = energy (J) required by single photon
Threshold frequency (f0) = minimum frequency (Hz) the incident light must have
Milikan's Contribution
Milikan conducted experiments involving the photoelectric effect. The main result of his work was an experimental way to measure Planck's constant.

Milikan graphed the kinetic energy of electrons as a function of the frequency. (kinetic energy is the responding variable (y) and frequency is the manipulated variable (x))

He found:
An electron was not released if the incident light did not meet the threshold frequency.

When the incident light was exactly at the threshold frequency the metal emitted and electron but it had ZERO kinetic energy

When the incident light was above the threshold frequency the photoelectron emitted had kinetic energy.

As the frequency of the incident light increases, the kinetic energy of the electron increased proportionally.

If the intensity is increased of a light that is ABOVE the threshold frequency, the current will increase too (more light = more photons = more electrons released)
Photoelectric effect formula
Photoelectric Effect Graph
Milikan's graph when testing the photoelectric effect was electron kinetic energy as a function of the intensity of light.

Manipulated variable = intensity (x)
Responding variable = electron kinetic energy (y)

For a linear graph the relationship is y=mx+b. Y is the dependent variable, m is the slope, x is the independent variable, and b is the y-intercept.

y=mx+b is related to Ekmax=Ephoton-W

Y-intercept = Work function (W)
X-intercept = Threshold frequency (fo)
m = Planck's constant
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