Photoelectric Effect

Shihab Uddin Ahmed | August 30, 2013

photoelectric effect photoApplication of photoelectric effect is everywhere around us. Digital camera, light meter in playground, solar panel, night vision device and many things are made using photoelectric effect. Albert Einstein is famous for his special theory of relativity. But he received his Nobel Prize for explaining the photoelectric effect not for his special theory of relativity. It was a question for many years, whether light consists of wave or particle.

Einstein’s explanation on photoelectric effect was based on particle theory. His explanation paved the way for acceptance of wave-particle duality of light. That means light consists of particles called photon and at the same time light is electromagnetic wave.

Explanation of photoelectric effect

Photon’s energy is, E=hν. Here h is Plank’s constant and ν is the frequency of light. So photon’s energy increases with the increment of frequency.

Mostly metals show this phenomenon. If a photon hits a metal, energy of the photon may be transmitted to an electron of the metal. There are two possibilities, one is electron will receive energy from the photon or it will not receive energy.

A portion of the energy provided by photon will be used to eject electron from the metal and remaining portion will be converted as the kinetic energy of the electron. Electron is bound with atom and a minimum energy is required to eject it from the metal. If the energy provided by the photon is equal or greater than the electron’s binding energy electron will be liberated. The liberated electrons are called photoelectron.

Extra energy will be converted to kinetic energy of the electron. If the photon’s energy and electron’s binding energy is equal, electron will just come out of the metal surface.

E=hν equation states that the energy of the photon depends on frequency. So if we increase the frequency of our light source, photon will get more energy and the electrons coming out of the metal surface will have more kinetic energy.

Increasing the intensity of light source doesn’t mean increasing the energy of photons. High intensity light means high number of photons is coming from the source. So by increasing the intensity we can change the number of electrons coming out of the metal surface. But the kinetic energy of the electrons remains the same.

Threshold frequency

Every metal have a characteristic binding energy of electrons. Usually alkaline metals are suitable for photoelectric effect experiment. But every metal shows photoelectric effect, we only need a minimum frequency. Because, a minimum frequency gives the minimum energy to eject electron. This minimum frequency is called threshold frequency.

A photon of threshold frequency will only eject electron, but that electron will not have kinetic energy.

As frequency of red light is lowest, photoelectric effect is not shown by red light.

Work function

Enrgy of photon, E=hν

Then the energy is used as, hν= hν0+Kmax

Kmax  is kinetic energy of electron. hν0  is used to liberate the electron, it depends on the type of the metal. It is called work function. Sometimes it’s denoted by W.

Inverse photoelectric effect

Sometimes X-ray effect is called as ‘Inverse photoelectric effect’, where high voltage is created to focus electrons on metal.

Why photoelectric effect is not supporting wave theory

If wave nature of light was true, kinetic energy of electron would depend on the amplitude of light, not the frequency. We could liberate electrons by increasing the intensity only. But that’s not true; we need to increase the frequency.

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