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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 determined using the

equation E = hf, where h is Plank’s constant,

6.626 x 10 –34 J/Hz. According to Einstein’s

theory an electron is ejected from the metal by a

collision with a single photon in the process, all the

photon energy is transferred to the electron and

the photon ceases to exist. However, the result is

the creation of a photoelectron. Since electrons

are held together in a metal by attractive forces,

some minimum energy Wo (work function) is

required to release an electron from the binding

force. If the frequency (f) of the incoming light

causes hf to be less than Wo, then the photons will

not have enough energy to emit any electrons.

However, if hf is greater than Wo, then the

electrons will be liberated and the excess energy

becomes the kinetic energy of the photoelectron,

allowing it to travel, creating an electric current.

Einstein’s theory uses the existence of a threshold

frequency to explain the photoelectric effect. A

photon with minimum energy hf is required to emit

an electron from the metal. Light with a frequency

greater than the threshold frequency (fo) has more

energy than required to emit an electron. The

excess energy again becomes the kinetic energy of

the electron, thus, Ek = hf – hfo. This equation is

known as Einstein’s Photoelectric Equation. An

electron cannot accumulate photons until it has

enough energy to break free; only one photon can

interacts with one electron at a time. In Einstein’s

equation hfo, is actually the minimum energy

required to free an electron. Not all electrons in a

solid have the same energy; most need more then

the minimum (hfo) to escape. Therefore, the

kinetic energy of the emitted electrons is actually

the maximum kinetic energy an emitted electron

could have. Einstein’s theory can be tested by

indirectly measuring the kinetic energy of the

emitted electrons. A variable electric potential

difference across the tube makes the anode

negative. Since, the anode rejects the emitted

electrons from the cathode, the electrons must

have sufficient kinetic energy at the cathode to

reach the anode before turning back. A light of

measurable frequency f, is directed at the cathode.

An ammeter measures the current flowing through

the circuit. As the opposing potential difference is

increased, the anode is made increasingly more

negative. At some voltage, called the stopping

potential, there is a zero reading from the ammeter

because the electrons do not reach the anode.

This is due to an insufficient amount of supplied

energy to the electrons. The maximum kinetic

energy of the electrons at the cathode equals their

potential energy at the anode. Emax = -qVo,

where Vo is the magnitude of the stopping

potential in volts (J/C), and q is the charge of the

electron (-1.60 x 10-19C). The joule is too large

a unit of energy to use with atomic systems,

therefore the electron volt (eV) is used instead. 1

eV = (1.60 x 10-19C) (1V) = (1.60 x 10-19C)

(V). Also, 1 eV = 1.60 x 10-19J. The results from

this experiment will show that higher frequency

radiation will have higher stopping potentials, and

lower frequency radiation will have lower stopping

potentials, holding true to Einstein’s hypothesis.

Conclusion The photoelectric effect revolutionized

the way the nature and behaviour of light is

understood. It also saw the dawn of modern

physics with the use of the Particle Theory, and it

catapulted Einstein to Nobel Prize-winning status.

Today, the phenomenon has many practical

applications such as alarm systems that activate

when the flow of light is interrupted.

Photoelectricity also helps explain the physics of

photosynthesis, by which plants make their own

food. It’s truly evident that the photoelectric effect

and its explanation played an important historical

role in science.

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