Photoelectric Effects

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Photoelectric Effects

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 Einsteins
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
Maxwells Theory of Electromagnetic Waves.
Hertz discovered that ultraviolet light discharged
certain electrically charged metallic plates, a
phenomenon that could not be explained by
Maxwells 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 Maxwells 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. Th...

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