Visual Quantum Mechanics The Next Generation

A large number of devices detect the presence of light. These range from the “switches”
which automatically turn on the street lights when it gets dark to the receiving end of the
scanners at the grocery store check out counter to the detectors in a CD player. All of
these devices work because light striking a material deposits energy in that material and
causes a change in the motion of the electrons. The detection of this change helps determine
how dark it is, how much your potato chips cost, and what sound should be produced
by the stereo.
The basic process behind all of these devices is the photoelectric effect. This effect was
first explained by Albert Einstein in 1905 and is cited as the reason for his Nobel Prize.
(When Einstein was awarded the Noble Prize, the Theory of Relativity was very controversial,
so it was not included as the reason.) We will study this photoelectric effect in this
experiment....


We have not studied the photoelectric effect in class, so you will need a little background
information to understand the measurements which you will need to complete. In this
experiment we will look at a situation similar to the one that Einstein originally explained.
It uses rather high-energy visible and ultraviolet light. In many devices today, particularly
CD players and grocery store scanners, the energy of the light is much lower.
The photons of the light source have the energy;
Ephoton = hfphoton h = 6.626 x 10-34 J•sec = 4.136 x 10-15 eV•sec
where E is the energy of the photon, f is the frequency of the light and h is Planck’s Constant.
The frequency of light can be found from the wavelength by:
f = c/λ c = 3.00 (10)8 m/sec
When an electron gets enough energy from a photon, the electron can be ejected from
the metal. However, the electrons are bound in the metal so some of this energy must
be used to overcome this binding. For bound electrons their maximum kinetic energy
will be less than the energy of the light photon. Because interactions with the metal will
differ from different electrons, not all of them will have the same kinetic energy even
though they have absorbed the same total energy from the photon. We relate the kinetic
energy of the ejected electron to energy of the photon as:
KEmax = Ephoton - Wo = hf – Wo
Here, Ephoton is the energy of a photon, h is Planck’s constant, f is the frequency of the
light, KEmax is the maximum kinetic energy of the electron released from the metal, Wo is
the energy needed to eject an electron from the metal.

Modern device that use the photoelectric effect are frequently called photocells. These
devices include a metal that will emit the electrons when light strikes it and another metal
plate. The energy of photoelectrons can be changed by placing a voltage across the two
metal plates. As the electron moves between these plates, it gains or loses energy which
depends on the voltage applied to the plates. (An energy gain occurs when the electron
is traveling toward the positive plate; a loss occurs when it is traveling toward the negative
plate.). In next part of this experiment we will use this fact to determine the kinetic
energy of the electrons.