THE PHOTOELECTRIC PHENOMENON --- A STUDY ON THE PARTICLE NATURE OF LIGHT

It is observed that when a light shines on a material there is an emission of electrons. This phenomenon is known as photoelectric effect, while the process is called photoemission. The electrons that gets emitted when this happen is called the photoelectrons. Don't mind the name, photoelectrons are the same as the normal electrons. The "photo" before their name just shows that the were emitted due to light.

I will really like you to allow your imagination to be very active in this discussion, because what we are about to discuss today is a bit abstract and can be fully understood with practicals, but if we can use our imaginations we can achieve the same result.

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If am to ask someone amongst us what he/she thinks that causes photoemission, he or she may try to explain that maybe when light incidents on a material, the light wave causes the electrons to vibrate and in the process energy is transferred from the light wave to the electron, thus the electrons gets energized. This now makes the electrons to possess the energy required for it to jump out of the material. He/she might even go further to tell us that a change in the intensity of the light will lead to changes in these electrons emmited from this material.

Though this answer sounds applausive it is not really true. In fact there was a time classical physicist thought the same because they felt that the idea was logical.

I will like us to revisit what they thought was the cause of photoelectric effect in 19th century. This will help us to appreciate the work Albert Einstein did on photoelectricity.

PHOTOELECTRIC EFFECT IN 19TH CENTURY

The Classical physicist of 19th century thought of light as a wave not as a particle, thus when they observed that when light is incidents on a material, electrons gets emitted, they tried to explain this phenomenon using the idea of light as wave. While seeking of a way to explain the phenomenon, they compared light waves with water waves and using that idea they predicted the following:

• When the light waves' amplitude increases, the kinetic energy of the emitted photoelectrons increases.

• And when the light waves' frequency is increases, the rate of electron emission (i.e measured current) increases too.

Like I said earlier, they made this prediction based on the idea that photoelectric effect is as a result of light waves, they then compared light waves with water waves assuming that both behave exactly the same. Let follow their line of thought and see how logical it was.

Imagine a ball that is sitting on a dock is been hit by a big wave. It is observed that the the ball moves a longer distance than it will move when it's hit by a small wave. A small wave can manage to move the ball only a few distance away. Acknowledging this, they concluded that for a light waves it is expected that a very high amplitude of light wave will result to high kinetic energy being impacted on the electrons. With that they got their first prediction.

Secondly, let's assume that the ball was being pushed consecutive by a large number of small waves. An experiment will show that the ball will move a greater distance away than when a big wave moves it only ones. With that they came to the conclusion that they got their second prediction.

But in science we know better than accepting things based on logical assertion, so those physicist decided to experiment on it. When they experimented on their theory they saw how wrong they were.

I bet that right now you will be asking questions like

"Are we trying to say that light is not a wave?"

"My physics teacher thought us about light wave, was he lying? "

That's not what I am trying to say. Let me explain.

REASONS WHY PHOTOELECTRIC EFFECT CAN'T BE EXPLAINED USING WAVE THEORY

Light has a dual nature, light can act as a particle and sometimes it can act as a wave. In the photoelectric phenomenon light acts as a particle.

Photoelectricity

For us to better understand this, we need to understand why the photoelectric effect is not a light wave phenomenon, but "light as a particle" phenomenon.

A wave is a disturbance which transmits energy through matter or space without a total displacement of medium.

If we tried to explain the photoelectric phenomenon using light waves, so many things won't add up. Let me just list out a few.

• If the intensity of a wave is increased, what it does is to increase only the amplitude of the wave. This doesn't add up to what is observed during photoemission. During photoemission, when the intensity of the light is increased, a corresponding increase is noticed on the number of electrons is noticed.

• In normal wave physics the energy of a wave is dependent on the amplitude. Thus, it is expected that at any frequency, electron can be emitted from a material, since all that the electron needs is energy for it to leave the metal, but that's not what happens during photoemission. During photoemission only lights of certain frequency can cause an electron to be emitted from a metal.

Because of these observations, scientists started experimenting and researching so as to understand what happens during photoemission.

Finally Albert Einstein had a breakthrough.

ALBERT EINSTEIN FINDINGS

He proposed that a beam of light is not just a wave propagation (unlike what most people thought as of that time), but rather as a collection of discrete wave packets ( which is called photons). Each of these photons has an energy of hv=E. Where

        h = Planck constant

        v = frequency 

        E = Energy 

I will like you to imagine these photons more like a particle.

Due to the energy this photons carries what happens is that those electrons in the material gets dislodged by these photons, but this can happen only when these photons reaches or exceeds a threshold frequency. It is also observed that when the emission of these electrons causes current to flow (photoelectricity). This flow can be detected by a microammeter.

To properly understand this, let me explain by using this example. Have you ever noticed while washing the plates in the sink that when you open the tap and allow the water to rush out into the dirty plate, if you switch OFF the tap and examine the plate you will observe that some of those stucked particles in the plate was knocked off by the force of the water.

Having observed this, try to unlock the tap slowly this time and allow the what to drip at a small rate. Now lift up the plate and examine it, you will observe that little or nothing was done in removing any of the stuck particle in the plate. Also the rate by which water flows out of the tap affects the way the dirty is washed off the plate. Using this analogy I bet you now agree that the rate or the frequency this photons incident on the material surface is going to determine if the electron gets knock off or not.

It was also observe through experiments that below a particular frequency known as the threshold frequency, no electron is emitted from the material. It doesn't matter the intensity or how long the material was exposed to light.

Let's try an imagine another scenario, remember that if the tap is allowed to keep flowing at a very small rate, let's assume this time the table is horizontal and lying, such that the water doesn't posses potential energy but flows directly into the plate (unlike the way our tap is positioned vertical to the plate), we will observe that no matter the amount of time, (assuming the water doesn't soften the dirt) the little drop would not be able to push the dirt away.

Having understood this, I will like us not to generally consider all the materials in the world, but rather consider only metals. This is because the photoelectric effect is more observable in metals (conductors) than in other non conductors.

Photoelectric effect is more observable in metals

If you are wondering it is so, Let me explain.

Remember that in our last section in Electrons, holes and electricity we tried to explain the reason why metals conduct. We understood from that discussion that metals possesses free electrons that can move about the way they like, and it is because of this freedom that the metals find it easy to conduct because only a little inertia force is required to start them moving in a direction. Unlike those of non conductors which requires energy to overcome the great energy that is binding the electrons to atom.

Thus it is quite easier for these free electrons of metals to be dislodged by the impingement of the photons than in other materials.

Don't forget that we said earlier that the intensity doesn't really matter, once the threshold frequency is reached, even at low intensity the material can eject electron.

Also the energy of the photon must be equal to the energy required to eject an electron from the metal (this energy is also called work function + the kinetic energy possessed by the electron when it's emitted.

There is something observed about the electrons during a photoemission. Have you ever tried paying back someone you are owing part of his money, only for him to give you back the money and tell you to hold it and bring the money when it's complete. I bet you have!. The person might tell you that his reason is that he doesn't want to spend the money, and he will prefer to have the money in bulk so as to be able to use it for something. The same thing happens in photoelectric emissions.

When the electron is been hit or impinged by the photon, the photon must do that at the right energy or else nothing will be achieved. Let try an imagine moving box of 200N weight by applying a 150N force. The 150N force doesn't move the box, but that doesn't mean that energy wasn't spent. The energy was received from the 150N by the 200N box, but since it is not enough, the 200N box doesn't retain the 150N force waiting for a 50N force to come and move it completely, rather what it does is to dissipate the whole 150N force out into the surrounding, then starts waiting for a force that is equal to or greater than 200N.

The same thing happens in electron, no matter the intensity, if the frequency (or the energy) of the photon is not enough at that moment to dislodge the electron, the electron doesn't store the energy but rather the energy is dissipated into the surrounding. This is called the "All or Nothing" principle.

For a photoelectric effect to occur the photons must have an energy that ranges from close to 0ev (for a negative electron affinity) to over 1Mev (for electrons in elements that have high atomic number). Mev stands for Mega electron-volt. Mega means 10^6.

Due to the level of freedom of the electrons in metals it only takes a few electron-volt which corresponds to the wavelength of visible or ultraviolet light.

After disproving the light wave theory of photoelectric effect, we now come to these conclusions.

• Electric current increases as the light amplitude increases.

• Electric current doesn't change with increase in light intensity.

• Kinetic energy of photoelectrons increases as light frequency increases.

• The kinetic energy of a photoelectron is not affected by an increase in amplitude.

APPLICATIONS OF PHOTOELECTRIC EFFECT.

• Solar energy is one of the promising source of energy in the world. Some of us do have solar panels in our houses. Solar panels are produced with photovoltaic cells that are made of semiconductors which produce light when exposed to sunlight.

Solar panel works on the principle of photoelectric effect

• Devices like scintillators operates using photoelectric effect. Scintillators are used to dictate radiation in a lab or radiation from cosmic forces.

• Some electronic devices like photocopier, phototransistors and photodiodes uses photoelectric effect. Even some calculators are powered by solar energy.

CONCLUSION

Our major sources of power on earth includes petrol, kerosene, diesel, radioactive materials e.t.c but their by products have been a major cause of pollution in our environment. Currently we are experiencing so much sunlight due to the degradation in our ozone layer caused by these harmful chemical0 substances in the atmosphere.

The worst part of the problem is that the number of automobiles in the road are increasing everyday, even the number of generators and power plants are increasing, leading to more problems. We need to think of another source of energy that won't pose threat to our environment.

Luckily, we have discovered a way that doesn't pose a threat to our environment. Using photoelectric effect, solar panels can generate enough electricity that can power buildings. We have even heard of cars that run on solar energy. This discussion was aimed at helping us understand the photoelectric phenomenon so as to harness the energy (photoelectricity) that can be generated from it. If we can channel our resources into developing this source of energy, it will really help in saving our dying planet.

REFERENCES

Why photoelectric effect can't be explained by using wave theory

Photoelectric effect wiki

Explaining photoelectric effect

What is photoelectric effect

Discovery and photoelectric principle

Quantum hypothesis explanation of photoelectric effect

Electron from matter and light from light

Dual nature of light

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