The PhotoElectric Effect.

Phenomena Explained.

Possibly the most complicated part of Unit 1, good to get your head around but easy to forget. Here are the cards to remind you of all the parts that don't make sense, as well as some Formula.

Starting with The Photoelectric Effect.

Next set of cards: Energy Levels and Photon Emission.

Have fun + Good Luck,

Oli B.

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The Definition.

The Photoelectric Effect is the process in which Electrons are Emitted when Light with a High Enough FREQUENCY is shined on any Metal.

For most Metals, this Frequency falls into the U.V Range.

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The Process.

1) Light or Electromagnetic Radiation falls onto a Surface of a Metal.

2) Free Electrons on the metal surface Absorb the Energy from the Light / EM Radiation.

3) The Electrons Vibrate faster by Conveting the Energy from the Light / EM Radiation into Kinetic Energy.

4) If enough of this Energy has been Absorbed and Converted into Kinetic Energy, the Bonds holding the Free Electrons to the Metal will Break and the Electrons will be Released.

This is called the Photoelectric Effect as the Electrons Emitted are called Photoelectrons.

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The Conclusions.

Conclusion 1:For any Given Metal, No Photoelectrons are Emitted if the Radiation has a Frequency Below a Certain Value (The Threshold Frequency).

Conclusion 2: The Photoelectrons are Emitted with a variety of Kinetic Energies Ranging from 0 to some Maximum Value. This value of Maximum Kinetic Energy Increases with the Frequency of the Radiation, NOT the Intensity of the Radiation.

Conclusion 3: The Number of Photoelectrons Emitted per Second is Proportional to the Intensity of the Radiation.

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The Phenomena.

Both Conclusion 1 and Conclusion 2 can't be Explained by the Wave Theory of Radiation.

According to the Wave Theory:

  • For a Particular Frequency of Light, the Energy carries is Proportional to the Intensity of the Beam.
  • The Energy carried by the Light would be Spread Evenly over the Wavefront.
  • Each Free Electron on the Surface of the Metal would gain a Small amount of Energy from each Incoming Wave.
  • Gradually, each Electron would gain Enough Energy to leave the metal.

So the problem is:

If the Light had a Lower Frequency (I.E carrying less energy) it would take Longer for the electron to gain enough Energy, but it would happen eventually. This doesn't explain the Threshold Frequency.

The Higher the Intensity of the wave, the more Energy would be Carried so More Energy should transfer to each Electron, this also means that the Kinetic Energy of the Electrons should increase with the Intensity. This doesn't explain that the Kinetic Energy Increases depending on the Frequency.

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The Explaination.

We can see that none of the information on the Wave Theory fits in with the Conclusions of the Photoelectric Effect.

So, Einstein came up with the idea (or Theory) that Light and Electromagnetic Waves comes in small packets at a time, known as Photons.

The Energy Carried by the Photons is:

E = hf = hc / wavelength

h being Planck's Constant = 6.63 x10 -34 Js

c being the Speed of Light in a Vacuum = 3.00 x10 +8 ms -1

Einstein saw Photons of Light as having a one-on-one, particle-like interaction with an Electron on the Metal's Surface.

The Photon would Transfer All of its Energy to that One, Specific Electron.

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The Photon's Explaination.

So, according to the Photon Model:

1) When Light hits the Metal Surface, the Metal is Bombarded by Photons.

2) If one of these Photons Collides with a Free Electron, the Electron will Gain Energy Equal to hf.

Before and Electron can leave the Metal Surface, it needs Enough Energy to Break the Bonds holding it to the Metal. This Value of Energy is known as the Work Function Energy (Symbol- Greek Phi) and its Value depends on the Metal.

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