3.1 The Photo Electric Effect Flashcards
What conduction eLectrons in a metal collide with
With each other and with the positive ions of the metal
What did Heinrich hertz discover about the found that sparks produced by radio waves by his spark gap detector
The sparks produced by radio waves were greater than those of ultraviolet
What is the photo electric effect
When electromagnetic radiation above a certain frequency was fired at a metal, electrons are emitted from the metals surface
First problem with the idea that light is a wave
Photoelectric emission doesn’t take place if the frequency of the incident electromagnetic radiation is below the threshold frequency (dependent on type of metal)
This means that the wavelength of the incident light must be less than a maximum value equal to the speed of light because speed of light/threshold frequency = wavelength
2nd problem with the idea that light is a wave
Number of electrons emitted per second is proportional to the intensity of the incident radiation (provided its greater than the threshold frequency)
If the frequency of the incident radiation is less than the threshold frequency no photoelectric emission will take place from that metal surface (no matter how intense the incident radiation is)
3rd problem with light being a wave
Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface (provided the radiation exceeds the threshold) regardless of intensity
What can the wave theory of light not explain
The existence of threshold frequency
Why photoelectric emission occurs without delay - according to wave theory, each conduction electron at the surface of the metal should gain some energy from the incoming waves regardless of how many waves arrive each second
Who was the photon theory of light put forward by
Einstein
What did Einstein assume about light
Light is composed of wave packets or photons, each of energy equal to hf
Where f is frequency of the light and h is the Planck constant
For electromagnetic waves of a wave length energy of each photon is
E=hf=hc/lambda
For electromagnetic waves of a wave length energy of each photon is
E=hf=hc/lambda
Where c = the speed of the electromagnetic waves
Explanation of the photo electric effect
When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf (energy of a light photon)
When can an electron leave the metal surface
If the energy gained from a single photon exceeds the work function of the metal
This is the minimum energy required by an electron to escape from the metal surface.
Excess energy gained by the photoelectron becomes kinetic energy
Maximum energy of an emitted electron
Ekmax = hf - work function
Rearrangement = hf=Ekmax+work function
How to work out threshold frequency of a metal
Emission can take place when Ekmax >0 or hf > work function
Therefore Fmin= work function / h
What is stopping potential (Vs)
Electrons that escape from metal plate can be attracted back to it if it is given a sufficient positive charge.
The minimum potential needed to stop the photoelectric emission is called stopping potential
What happens at this stoping potential
The maximum kinetic energy of the emitted electron is reduced to 0 because each emitted electrons must do extra work equal to e x Vs to leave the metal surface
Hence the maximum kinetic energy is equal to e x Vs 
What experiment was evidence for Einstein’s theory
Robert Milllikan measured the stopping potentials for a range of different metals using different light frequencies.
His results fitted Einstein’s photoelectric equation closely.
After being peer reviewed and checked scientists excepted light as photons.
Threshold frequency
Minimum frequency of light that can cause photoelectric emission
Work function
Minimum amount of energy needed by an electron to escape from a metal surface
Stopping potential
The minimum potential needed to stop the photoelectric emission
