# The Story of Photoelectric Effect

I have a story to tell, the story of the photoelectric effect. Apart from the story, I have a few demonstrations, questions and their answers. The main characters of the story are Heinrich Hertz, Phillip Lenard, Albert Einstein and Robert Millikan. Three of them got the Nobel Prize. The story is based on the original work of these scientists. I promise you that you will learn many things through this interesting story.

Let me take you back a hundred and fifty years. A scientist in England was working hard to establish relationships among electricity, magnetism, and the light. He believed that these three are manifestations of a single phenomenon. He came up with four beautiful equations. These equations are known as Maxwellian equations. He was James Clerk Maxwell, a brilliant theorist. According to Maxwell, accelerating charges shall produce electromagnetic waves. Today everyone knows about electromagnetic waves but that was not the case in 1873. Scientists were not aware of how to produce them or how to detect them. Maxwell died at the age of 48 years, much before radio waves were discovered by a German scientist Heinrich Hertz.

Electromagnetic waves took 14 years from their prediction by Maxwell to their detection by Heinrich Hertz. By the way, the unit of frequency is Hertz. It is named after Heinrich Hertz. You are able to hear me because your ears are sensitive to audible frequencies from 20 Hertz to 20000 Hertz. You are able to see me because your eyes are sensitive to electromagnetic waves from 400 trillion Hertz to 800 trillion Hertz. Let me show a simple demonstration that produces and detects electromagnetic waves.

You must have seen a little spark in the gas lighter. Gas lighter can produce up to 1000 Volt which can give you a little shock. I have taken two wires from this lighter to get 1000 volt. These wires are connected to two bicycle spokes. The spokes are made very sharp at their end. When I press the lighter, you can see a little spark between two spokes. The air between these two ends gets ionized by the high voltage. The charge flow from one spoke to the other. These accelerating charges produce electromagnetic waves as predicted by Maxwell. These waves travel all around me. And they also reach another pair of spokes. There is a small Neon bulb connected between these spokes. The neon bulb glows whenever spark is produced between these spokes. The electromagnetic waves are produced here and detected here. It is very difficult to see the spark.

And his problems were more severe. He was not having a gas lighter or the neon bulb. He used an electrical circuit to generate high voltage. In place of a neon bulb, he used a ring with two metal balls at the end. Whenever electromagnetic waves reach this ring, a spark is produced in these balls. The spark was difficult to see. When he illuminated these balls with ultraviolet rays, the spark became bigger which was easy to see.

The light consists of many colors from violet to red. These colors have different frequencies. The red light of low frequency - around 400 tera hertz. The violet light is of higher frequency - around 800 tera hertz. Apart from visible light, there are infrared radiations of frequencies less than that of red. And there are ultraviolet radiations of frequencies more than violet. The ultraviolet radiations are of high energy. These are harmful to our skin and eyes. UV rays are also produced by the sun but the ozone layer protects us from the harmful UV radiations.

When these high energy UV rays fall on the metal surface, they eject electrons. These electrons are negatively charged and they ionize the air. The discharge length increased and it was easy to see. The ejection of electrons from the metal surface by the high energy radiation is called the photoelectric effect. Hertz discovered this effect but he could not study it further. He died at a young age of 36 years. His daughters were not married. We don’t have any descendents of Hertz today. But you can see his name on almost every electrical equipment. The unit of frequency is hertz. It is written as Hz, uppercase H lowercase z.

His work was carried forward by his student Wilhelm Hallwachs. He used an electroscope to study the photoelectric effect. An electroscope is used to study charges. When you comb your hair - there is an exchange of charges between hair and comb. One of them gets positively charged and another is negatively charged. You can lift pieces of paper with a comb after rubbing it with hairs. The electroscope is used to study these charges in a more systematic way. You can make an electroscope at home.

This is a homemade electroscope. Take a plastic or glass bottle and insert a wire through the cap. Bend the lower end of the wire as a hook. Cut two pieces of aluminium foil 1 cm by 3 cm and hang them in the hook. These foils or leaves should be free to move. Put an aluminium or metal plate on the other end of the wire. Your electroscope is ready.

It is a glass rod. When I rub it with my clothes, it gets positively charged. When the rod is brought close to the electroscope, the leaves diverge. Let us try to understand what is going on.

To begin with, the aluminium plate and leaves were neutral. When a positively charged rod is brought close to the aluminium plate, it attracts negative charges towards it. The negative charges accumulate at the plate. Some negative charges move from the leaves to the plate because they are connected by metallic wire. Thus, both leaves become positively charged. Like charges repel and hence leaves diverge.

Now, if I touch the aluminium plate, the leaves converge. They become neutral. Their positive charges are neutralized by the negative charges that flow from my body to the leaves. But the aluminium plate is still negatively charged because these charges are bound by the glass rod. If I remove my finger, and the rod, the negative charge of the aluminium plate is redistributed. Some of the negative charge moves towards the leaves. And the leaves again diverge. The electroscope is negatively charged. There is an excess of electrons on it.

This is a UV lamp. It produces ultraviolet radiation. These radiations are of high frequency and high energy. You should not expose your body to these radiations. I made this electroscope - it is the same but horizontal. There are leaves, wire, and aluminium disc. I am using a plastic sheet to increase the contrast so that you can focus on the leaves. Let me charge it. Rub the glass rod, bring it close to the aluminium disc, touch with finger and remove. The electroscope is negatively charged. It has an excess of electrons. What will happen if I illuminate the aluminum surface with a UV lamp? UV rays will eject electrons and the electroscope will become neutral. Leaves shall converge. Let us see whether this happens or not. Yes it does. It demonstrates the photoelectric effect. Similar study was done by Hallwachs. These are observations, a qualitative study.

A detailed study of the photoelectric effect was carried out by another German scientist Phillip Lenard. He used a sophisticated setup to do measurements. It tooks 3 years from 1899 to 1902 to do measurements and analyze the results. In 1905, Lenard received the Nobel prize for this work. I have a model to understand his experiment.

There is a metal plate at this end. It is illuminated by radiations like radiations from this UV torch. The radiation ejects electrons from the metal surface. These electrons are ejected with different velocities - some of them move fast and some slower. There is another plate on the other end. It will repel electrons if it is at a negative potential. The potential can be controlled by batteries. If the negative potential is large enough then even the fastest electron cannot reach this plate. This potential is called stopping potential. It stops electrons movement from one plate to the other. Lenard observed that stopping potential depends on the frequency of light - it is more for high frequency radiations. Thus, electrons ejected by high frequency radiations move much faster - they have more kinetic energy. It is independent of the intensity.

If the plate is at the positive potential then it attracts the electrons. There is movement of charges in the circuit. A current flows from one plate to the other. He measured the current with a sensitive ammeter. He observed that the current is independent of the frequency. It depends on the intensity of the light. More the intensity, more is the current. A 10 watt lamp gives more current than a 5 watt lamp.

There were difficulties in explaining these observations. The wave theory prevalent at that time was unable to give convincing explanations.

In 1905, Albert Einstein published a paper that gives a new perspective of radiation. It explains Lenard observations. According to this view, light of a particular frequency consists of a large number of identical particles. All particles have equal energy. Total energy is the sum total of the energy of these particles. Einstein called these particles light quanta. Today, they are known as photons. In a photoelectric effect, one photon interacts with an electron. It transfers its energy to the electron. The electron loses some of its energy by doing work against the binding force of the metal. The energy lost in doing this work is known as the work function of the metal. The remaining energy is the kinetic energy of the electron. The maximum kinetic energy of the electron is Kmax=h v - W.

Einstein’s theory was not accepted by the Physicist at that time. They were not ready to accept this new way of looking at the light. Because wave nature of light was well established in experiments like diffraction and interference and Maxwell gave a solid theory for electromagnetic waves. In physics, you need experiments either to prove or disprove a theory. Rober Millikan, a brilliant experimentalist in America, decided to disprove Einstein theory by an experiment. He designed a marvelous experiment to disprove einstein. But he failed to do so. His experimental results were in agreement with Einstein's equation.

Einstein was awarded the Nobel prize in 1921 for his explanation of the photoelectric effect. A new way of looking at light. Two years later, Millikan also got Nobel prize for his experimental findings..

Maxwell gave the wave picture of light and Einstein gave its photon picture. The Hertz experiment not only established Maxwell theory but it also opened the door for Einstein theory. The light shows different properties to different observers - sometimes it behaves as waves and sometimes as particles. It reminds me of Tulsidas ji who said “जाकी रही भावना जैसी प्रभु मूरत देखी तिन तैसी”. WIth that I end the story.