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The Photoelectric Effect

Philipp Eduard Anton von Lenard showed that increasing 
light intensity increased the photoelectric current but 
not the critical voltage needed to halt the current flow.
Philipp Eduard Anton von Lenard showed that increasing
light intensity increased the photoelectric current but
not the critical voltage needed to halt the current flow.

Experimental Evidence

In 1902 Philipp Eduard Anton von Lenard performed an experiment designed to show how the energy of the emitted photoelectrons varied with the intensity of the light.

A photoelectron is an electron that is emitted from a metalic surface in response to stimulation by light falling on that surface.

Lenard set up an electric circuit with a gap consisting of two metalic surfaces facing each other through a vacuum. The separation of the plates was sufficient to prevent current flow across the gap under normal circumstances.

However when one of the plates was illuminated with a carbon arc light he found he could generate a current across the gap and hence within the circuit.

This was no real surprise, since Lenard knew that light was energy and that when energy is transferred to the electrons on one plate it could give them the energy boost they needed to allow them to escape one plate and travel across to the other.

Lenard added a twist to his experiment by applying a resisting voltage across the gap between the plates. Now he could measure the voltage required to halt the flow of electrons triggered by the light from his carbon arc lamp.

These are the observations he made using this apparatus.
  • Doubling the light intensity doubled the number of electrons emitted, but did not affect the value of Vo required to stop the flow of electrons from the surface altogether;
  • Changing the color of the light impinging on the metalic plate did effect the value of Vo required to stop the flow of electrons ... shorter wavelengths of light required higher values of Vo to stop the current.

Q: If light is energy why wouldn't a more intense light transfer more energy to the electrons and hence increase Vo?

Q: Why would the color of the light effect the value of Vo whereas the intensity of light does not?

        
 
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