4.1 Why Photons Flashcards
What is a black body?
Something that emits all possible wavelengths for its temperature (perfect emitter)
What was the ultraviolet catastrophe?
Relationship between the intensity and wavelength of emitted electromagnetic radiation for a black body.
Intensity should keep rising as wavelength decreases.
Drops in the ultraviolet section.
How did Max Planck resolve the ultraviolet catastrophe problem?
Electron oscillations in hot bodies could only take certain discrete values given nhf, where n is an integer, h is the Planck constant and f the frequency of the radiation- this is called quanta.
Equation for the way intensity varied with wavelength for the black body radiation.
What is the photoelectric effect?
The emission of electrons from a metal surface when electromagnetic radiation is incident on it.
“If a glass plate is inserted between the ultraviolet lamp and the zinc plate, the electron emission ceases.
If an ordinary filament lamp (emitting UV light) is used, the electroscope does not discharge.”
What do these observations show?
That there is a threshold frequency below which the photoelectric effect does not occur.
What did Einstein’s explanation of the photoelectric effect consist of? (brief)
The idea that the energy in electromagnetic radiation can only be transferred in well defined ‘lumps’.
The energy is said to be quantised and a quantum of electromagnetic radiation is called a photon.
What is the equation for photon energy?
E = hf
f: frequency of the radiation
h: Planck constant which has a value of 6.63 x 10^-34 J s.
What is the equation for the speed of electromagnetic radiation?
c = f x wavelength
What was Einsteins full explanation of the photoelectric effect?
Electrons are bound to a metal surface rather like gravity holds us to Earth.
To remove an electron from a metal surface a minimum amount of energy is needed.
This is called the work function φ.
An electron that is more tightly bound to the surface needs more energy than this.
The energy to remove an electron has to be delivered at one instant- an electron cannot store energy up a bit at a time.
When hf < φ…
The photon has sufficient energy to liberate an electron.
When hf = φ…
The electron can just leave the surface and f in this case is the threshold frequency.
When hf > φ…
There is energy to spare and this becomes kinetic energy of the electron.
What was Einsteins photoelectric equation?
hf = φ + E
What is meant by the term work function?
The minimum amount of energy required to remove an electron from the surface of a metal.
The wave theory of light cannot adequately account for the instantaneous emission of electrons from a metal surface that is illuminated with low intensity electromagnetic radiation.
Explain why, according to the wave theory, there should be a delay in the emission of photoelectrons.
Low intensity is low energy.
Wave energy would be absorbed continuously/gradually/over an area.
Wave energy could accumulate.
Photoelectron is released when energy accumulated is equal to work function.
