AP® Physics 1 & 2

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Photoelectric Effect: Red Light, Green Light


Red light shines on sodium metal and no current flows through the circuit.

The light is changed to a green light exactly at the threshold frequency for sodium of ${ f }_{ 0 }=5.706{ \times { 10 }^{ 14 }\text{ Hertz }}$ but still no current is observed.

"Created for Albert.io. Copyright 2016. All rights reserved.”

Which of the following is the best explanation of this observation?


The electrons are ejected from the sodium at the threshold frequency, but the energy provided is all used to release the electrons. None is left over to provide kinetic energy for the electrons to move and produce a current.


The electrons will be ejected depending on the intensity of the green light. Current is directly proportional to the light intensity. The experimenters must increase the brightness of the light to observe a current.


The electrons emitted will result in a higher work function for the sodium since they have more energy.


The current is only produced with frequencies less than the threshold frequency and stops when the threshold is reached. That is why it is called the threshold.