16-11-2012, 06:20 PM
Photoelectric Effect
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What is Photoelectric effect?
A photoelectric effect is any effect in which light energy is converted to electricity.
Quick Overview:
When a photon of light hits on the surface of a metal then ejection of electrons from that surface takes place
In the photoelectric effect, mainly we hit target with EM radiation and electrons fly out!
The electrons ejected from the target are called “photoelectrons”
Set Up:
An adjustable voltage is applied. Voltage can be forward or reverse biased (which slows down the electrons)
Photoelectrons return to cathode through an ammeter which records the current
Working:
• Incident light triggers the emission of (photo)electrons from the cathode having certain required frequency(Threshold).
• Some of them travel toward the collector (anode) with an initial kinetic energy.
• The applied voltage V either accelerates (if positive) or decelerates (if negative) the incoming electrons.
• The intensity I of the current measured by the ammeter as a function of the applied voltage V is a measurement of the number of photo- electron gets ejected.
Some Experimental Results:
If photoelectrons get ejected when we shine monochromatic light on the target, the current increases when we increase the intensity (brighter light = more photoelectrons)
BUT…below a “cutoff frequency” no photoelectrons get ejected no matter how great the intensity of the incident radiation
AND…for frequencies above the cutoff, decreasing the radiation to very low intensities does not completely eliminate the production of photo electrons
Laws of Photoelectric effect:
Depending on the metal and the frequency of the light, the number of photoelectrons released is proportional to the intensity of the light.
There is a threshold in all metallic elements that a certain frequency of light must have in order to emit photoelectrons this is called the threshold frequency.
The maximum amount of energy that a photoelectron can emit is dependent on the frequency not the intensity of the light.
So How Do Solar Cells Work?
If we take silicon as solid state material of solar cell.
Silicon is a semi-conductor and a insulator, these properties create a gap called “ bandgap” between electrons. Electron can not stay in the bandgap they must gain or lose energy forcing the electron further away from the nucleus or closer to it.
When the sunlight hits the silicon some of the electron gain enough energy to jump the bandgap and become free to conduct electricity.