12.1 The Quantum Nature Of Radiation Flashcards
What is the photoelectric effect?
The emission of electrons from a metal surface when electromagnetic radiation is incident on the surface. This is evidence for the behaviour of light as photons that is as particles or quanta of energy.
What is the work function?
The minimum energy required to eject an electron from a metal.
What is the stopping potential?
The potential at which the photocurrent in a photoelectric experiment becomes zero.
How can the photoelectric effect be demonstrated?
A sensitive galvanometer G measures the current flowing in the circuit. With battery connected data can be obtained for the current for negative voltages to obtain data fro positive voltages, the polarity of the battery must be reversed. As the applied voltage is made more and more positive the number of electrons attracted to the collecting plate increases and so the current increases. As the voltage is made more and more negative the current becomes zero at what is called the stopping potential. At more negative values of the voltage no more electrons are emitted from the photo surface so the stopping voltage is a measure of the maximum kinetic energy of the emitted electrons Emax = eVs.
What are the key experimental facts?
The kinetic energy of the emitted electrons increases with increasing light frequency.
The kinetic energy of the emitted electrons does not depend on the intensity of the light. The light intensity only affects the number of electrons emitted ie the photocurrent.
Below a particular value of the light frequency called the threshold frequency, no electrons are emitted at all, no matter how high the light intensity.
Electrons are emitted within 1 nanosecond of the incidence of light on the surface, thus with essentially no time delay.
What are the problems with the facts?
The experimental facts contradict the classical theory of light as waves.
What did Einstein predict about light?
Einstein suggested that light is made up of particles called photons - bundles or quanta of energy. In electron emission one electron absorbs one photon of energy E=hf where d is the frequency of the light. If the minimum energy needed for the electrons to escape from the metal is called the work function then the electron will be emitted with kinetic energy Ek=hf - work function.
What are the important parts of the graph if the maximum kinetic energy of the emitted electrons is plotted against light frequency?
The graph will be a straight line.
Th gradient of the line will be h (Planck’s constant)
The y intercept will be the work function.
A different photo surface will give a different but parallel straight line.
