3.1 The Photoelectric Effect Flashcards
What did Heinrich Hertz find when he discovered how to produce and detect radio waves?
The sparks produced in his spark gap detector when radio waves were being transmitted were stronger when ultraviolet radiation was direacted at the spark gap
Effect known as photoelectric effect
What observations were made about the photoelectric effect after Hertz’s discovery:
- Photoelectric emission of electrons from metal surface does not take place if frequency of incident electromagnetic radiation is below certain value (threshold frequency). Min. frequency depends on type of metal. Wavelength of incident light therefore less than max. value equal to speed of light divided by threshold frequency
- Number of electrons emitted per second is proportional to intensity of incident radiation provided that frequency is greater than the threshold frequency. If frequency of incident radiation is less than threshold frequency, no photoelectric emission from that metal surface can take place no matter how intense incident radiation is
- Photoelectric emission occurs without delay as soon as the incident radiation is directed at surface, provided frequency of radiation exceeds the threshold frequency and regardless of intensity
What can the wave theory of light not explain
Either the existence of a threshold of frequency of why photoelectric emission occurs without delay.
According to wave theory, each conduction electron at surface of metal should gain some energy from incoming waves regardless of how many waves arrive at each second
Who put forward the photon theory of light
Einstein in 1905
What did Einstein assume with the photon theory of light
That light is composed of wavepackets or photons, each of energy equal to hf where f is the frequency of the light and h is the Planck constant.
Energy of a photon =
hf
Where h is the Planck constant and f is frequency
For electromagnetic waves of wavelength λ , energy of a photon E =
E = hf = hc / λ
Where c is the speed of the electromagnetic waves
What did Einstein say to explain the photoelectric effect
When light is incident on a metal surface, an electron at surface absorbs a single photon from the incident light and therefore gains energy equal to hf where hf is the energy of a light photon
An electron can leave the metal surface if the energy gained from single photon exceeds work function φ of the metal
What is the work function of a metal
Symbol φ
The minimum energy needed by an electron to escape from the metal surface.
Excess energy gained by the photoelectron becomes its kinetic energy
The max. kinetic energy of an emitted electron is:
Ekmax = hf-φ
Rearranging this equation gives:
hf = Ekmax + φ
Emission can take place from a metal surface provided that…
Ekmax>0 or hf >φ, so the threshold frequency of the metal is fmin=φ/h
How can the electrons that escape from a metal plate be attracted back to it
By giving the plate a sufficient positive charge.
What is the stopping potential Vs
The minimum potential needed to stop photoelectric emission
At this potential, the max. kinetic energy of the emitted electron is reduced to zero because each emitted electron must do extra work equal to e*Vs.
Who gathered conclusive experimental evidence for Einstein’s photon theory
Robert Millikan
How did Robert Millikan gather conclusive experimental evidence for Einstein’s photon theory
Measured the stopping potential for a range of metals using light of different frequencies. His results fitted Einstein’s photoelectric equation very closely.
The results were checked and it was accepted that light consists of photons
