the photoelectric effect and describe features of atomic spectra
Answers
Around the turn of the twentieth century, it was observed by a number of physicists (including Hertz, Thomson and Von Lenard) that when light was shone onto a metal plate, electrons were emitted by the metal. This is called the photoelectric effect. (photo for light, electric for the electron.)
The characteristics of the photoelectric effect were a surprise and a very important development in modern Physics. To understand why it was a surprise we need to look at the history to understand what physicists were expecting to happen and then understand the implications for Physics going forward.
Einstein's model is consistent with the observation that the electrons were emitted immediately when light was shone on the metal and that the intensity of the light made no difference to the maximum kinetic energy of the emitted electrons.
The energy needed to knock an electron out of the substance is called the work function (symbol \(W_{0}\)) of the substance. This is a characteristic of the substance. If the energy of the photon is less than the work function then no electron can be emitted, no matter how many photons strike the substance. We know that the frequency of light is related to the energy, that is why there is a minimum frequency of light that can eject electrons. This minimum frequency we call the cut-off frequency, \(f_0\). For a specific colour of light (i.e. a certain frequency or wavelength), the energy of the photons is given by \(E=hf=\frac{hc}{\lambda}\), where \(h\) is Planck's constant. This tells us that the \(W_{0} = hf_0\) .
Work function
The minimum energy needed to knock an electron out of a metal is called the work function (symbol \(W_{0}\)) of the metal. As it is energy, it measured in joules (J).
Energy is conserved so if the photon has a higher energy than \(W_{0}\) then the excess energy goes into the kinetic energy \({E}_{k}\) of the electron that was emitted from the substance.
The excess over and above the binding energy is actually the maximum kinetic energy the emitted electron can have. This is because not all electrons are on the surface of the substance. For electrons below the surface there is additional energy required to eject the electron from the material which then cannot contribute to the kinetic energy of the electron.
The theory of the photoelectric effect must explain the experimental observations of the emission of electrons from an illuminated metal surface. ... The number of electrons also changes because of the probability that each photon results in an emitted electron are a function of photon energy.