Photoelectric Effect (Quantum Theory of Light)

• Einstein (1905) created the quantum theory of light, which states that electromagnetic radiation traveled as tiny packets of energy called quanta, or photons, that behaved like particles, rather than continuous waves.

• Einstein's work on quanta of light provides the foundation for an revolutionary new idea: our whole universe is composed of tiny, discrete chunks of energy and matter. 

The photoelectric effect is perhaps the most direct and convincing evidence of the existence of photons.

Albert Einstein received the Nobel prize in physics in 1921 for explaining the photoelectric effect and for his contributions to theoretical physics.

What is the Photoelectric Effect?

• A photon with energy strikes an electron and ejects it from the piece of metal. (hf)

• The energy does work to remove electron from metal. The work energy is converted into kinetic energy (KE) of the ejected electrons.

It's been determined experimentally that when light (a photonof energy) strikes a metal plate, electrons will be ejected from the metal.

The energy does work to remove electrons from metal. The work energy is converted into kinetic energy of the ejected electrons.

The Photoelectric Effect

Think about hitting a ball into outer space.

If you don't hit it hard enough, it will just come back down. No matter how many times you hit it.

If superman hit it, he could get it into space.

Similarly, no matter how many photons strike the metal, if none of them has sufficient energy to eject an electron from a metal atom, you won't get a current.

If the energy the taken up by the electron is sufficient to allow it to be released from the metal atom, you will get a current.

Here the photon is absorbed by the electron and ceases to be a particle.

Photoelectric effect

Shine light rays on metal --> produces current

Increase intensity of light rays (total energy) --> current goes up

The strange thing about this phenomenon, is that a certain minimum frequency of light is required before any current is detected.

The higher the frequency of light - the higher the energyof the photon

Higher intensity light has more photons, and so will knock out more electrons.

However, if the frequency of the light is such that a single photon is not energetic enough to release an electron from the surface, then none will be ejected no matter how intense the light.

One would expect the energy of the emitted electrons  to depend on the intensity of the light -- but it does not.

In Einstein's model, increasing the intensity of the light would cause greater numbers of electrons to be ejected, but each electron would carry the same average energy.

Increasing the frequency, rather than the intensity, of the incident radiation would increase the average energy of the emitted electrons.

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