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