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The Basic Idea of Photoelectric Effect (page 3)

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Author: Jerry Silver

Why It Works

Light contains energy based on its frequency. The frequency of visible light is lower than that of ultraviolet light and it does not have enough energy to free electrons from a metal, such as zinc. As the frequency of the light increases, the energy each photon carries is raised above the threshold required to free electrons from the zinc.

Photoelectric Effect

The work function of a metal is a measure of how tightly electrons are held by the atoms of the metal. If the photon energy is greater than the work function of the metal, electrons are released. If the freed electrons encounter a stopping voltage (stopping potential), the amount of extra energy above the work function can be determined.

This can be summarized by the equation:

KE = Ephoton + W

where KE is the kinetic energy of the freed electron (measured by the amount of voltage required to stop the electrons).

Ephoton is the energy carried by the photon. W is the amount of energy just to free one electron from the metal with no extra energy to get it moving.

The energy in a photon was given by:

Ephoton = hf

where h is Planck's constant and fis the frequency of the light.

Other Things to Try

A good software simulation of the results of this experiment can be found at http://phet-web.

The Point

The key concept underlying this experiment is that light energy comes in specific amounts or packages called quanta or photons. These photons cannot be broken up into smaller units. The higher the frequency of the light, the greater the amount of energy contained in one photon. If a photon has enough energy to release an electron, an electric current can flow; otherwise, below that threshold, no energy will flow. The more energy the photon has, the more kinetic energy the electron processes when it is released.

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