Effect of intensity and frequency of incident light
The velocity, and therefore the energy, of the emitted electron depends on the frequency of the incident light.The higher the frequency of the wave, the more energy each emitted electron has.
The number of electrons emitted per second depends on the intensity of the incident light.
The brighter (more intense) a light source, the more photons it gives out per second. Einstein’s Photoelectric Law
LAWS AND PRINCIPLES Einstein’s Photoelectric Law The energy of the fastest electron emitted is the difference between the energy of the photon and the work function of the metal.
The reason for the reference to the ‘maximum velocity’ is that electrons which are more tightly held in the metal will be emitted with a lesser speed.
Threshold frequency THRESHOLD FREQUENCY
The threshold frequency (f0) is the minimum frequency needed to cause photo- electric emission.
The value of the threshold frequency is equal to the work function divided by Planck’s constant.
WORK FUNCTION
hfo work function, h Planck’s constant, f0 threshold frequency
PHOTOELECTRIC LAW
hf 1 2 mv max
2
h Planck’s constant f frequency of the EM wave
work function of the metal
m mass of an electron
vmax maximum velocity of any of the
emitted electrons
SAMPLE PROBLEM 21E
A piece of zinc metal is illuminated with ultraviolet light of wavelength 240 nm resulting in the emission of electrons from its surface.The work function of zinc is 4.3 eV. Calculate: (i) the threshold frequency of zinc (ii) the maximum velocity of an emitted electron.
(Planck’s constant 6.6 1034 J s; speed of light 3.0 108 m s1; 1 eV 1.6 1019 J, mass of an electron 9.1 1031 kg.)
