The Photoelectric Effect

Introduction The
Quantum Theory was the second of two theories
which drastically changed the way we look at our
physical world today, the first being Einstein’s
Theory of Relativity. Although both theories
revolutionized the world of physics, the Quantum
Theory required a period of over three decades to
develop, while the Special Theory of Relativity
was created in a single year. The development of
the Quantum Theory began in 1887 when a
German physicist, Heinrich Hertz, was testing
Maxwell’s Theory of Electromagnetic Waves.
Hertz discovered that ultraviolet light discharged
certain electrically charged metallic plates, a
phenomenon that could not be explained by
Maxwell’s Wave Theory. In order to explain this
phenomenon termed the photoelectric effect,
because both light and electricity are involved, the
Quantum Theory was developed. The
Photoelectric Effect Maxwell’s work with the
Theory of Electromagnetic Waves may seem to
have solved the problem concerning the nature of
light, but at least one major problem remained.
There was one experiment conducted by Hertz,
the photoelectric effect, which could not be
explained by considering light to be a wave. Hertz
observed that when certain metals are illuminated
by light or other electromagnetic radiation, they
lose electrons. Suppose we set up an electric
circuit. In this circuit the negative terminal of a
battery has been connected to a piece of sodium
metal. The positive terminal of the battery is
connected through a meter that measures electric
current, and to another piece of metal. Both of
these metal plates are enclosed in a sealed glass
tube in which there is a vacuum. When there is no
light illuminating the sodium plate, no current will
flow, and therefore there is no reading on the
meter. A reading on the meter will only occur
when electrons are liberated from the metal
creating a flow of electric current. However, if the
sodium plate is exposed to light, an electric current
will flow and this will register on the meter. By
blocking the light from illuminating the sodium
plate, the current will then stop. When the amount
of light striking the plate is increased, the amount
of current also increases. If various colours of light
are tested on the sodium plate it will be discovered
that violet and blue light causes current flow.
However, colours of light toward the other end of
the spectrum (red) do not result in a flow of
electric current when they illuminate the sodium
plate. The electrons will only be emitted if the
frequency of the radiation is above a certain
minimum value, called the threshold frequency
(fo). The threshold frequency varies with each
metal. When the sodium plate was exposed to
high frequency light, electrons were emitted and
were attracted to the positive terminal, causing a
flow of current. However, when a low frequency
light was used no electrons were emitted and
therefore there was no current. Observations of
the Photoelectric Effect 1. Current flows as soon
as the negative terminal is illuminated. 2. High
frequency light causes electrons to be emitted from
the sodium, however, a lower frequency light does
not. 3. The energy of the emitted electrons does
not depend upon the intensity (brightness) of the
light, it is dependent on the frequency of the light.
A higher frequency of light causes higher energy
electrons. 4. The amount of current that flows is
dependent upon the intensity (brightness) of the
light. Prior to the 1900’s light was considered to
be wave-like in nature. This was due to the
success of Maxwell’s Electromagnetic Theory.
However, much of the phenomenon observed
during the photoelectric effect was in contradiction
to the Wave Theory of Light. For instance, the
energy contained in electromagnetic waves, and
the amount of energy that would strike a sodium
electron can be calculated. Such a calculation
shows that an electron could indeed gain enough
energy to be liberated from the sodium, but only
after the sodium had been illuminated for several
hours. However, this was not the case for
photoelectricity, in which the electrons are freed
instantly. The Electromagnetic Theory sustains that
light waves carry energy whether they are of high
or low frequency. Therefore, the frequency of light
should not be a factor in the emitting of electrons.
Once, again the photoelectric effect contradicts
the Wave Theory. In the photoelectric effect only
high frequency light can cause electrons to be
emitted no matter how long the light is shined. The
photoelectric effect was a major roadblock in the
way of total acceptance of the Wave Theory of
Light. Einstein’s Theory In 1905, Albert Einstein
published a revolutionary theory that explained the
photoelectric effect. According to Einstein, light
and other forms of radiation consist of discrete
bundles of energy which were later given the term
‘photons’. The energy contained in each photon
depends on the frequency of the light in which they
are found. The energy of the emitted
photoelectron can be