Light is an em wave and it also behaves as a 
particle
We saw that in the photoelectric effect.  
Light could behave as a particle.
 If nature is symmetric then a particle like an 
electron or a proton should also behave like a 
wave. 
Great thinking – obviously not mine but this is 
what de  Broglie proposed. 
Now lesser mortals like you and me would say ok 
I accept since such a great scientist is saying this 
but has anyone seen this happen.
yes
Davisson and Germer’s experiment did  provide 
experimental confirmation for what De Broglie 
said. 
Their experiment involved diffraction with 
electrons. You know that diffraction is a wave 
phenomena so now
we are talking of particles exhibiting wave 
behaviour and that is exactly what we are 
looking for.
First let’s take a look at the experimental set up. A 
piece of nickel was placed 
A piece of nickel was placed and a beam of 
electrons were directed at its surface. 
beam of electrons is directed at the target
The electrons were emitted from a heated 
filament and they were accelerated by the 
electrodes in the electron gun 
and then they are directed at the crystal. I haven't 
drawn that part. I've just taken a beam of 
electrons.
The number of electrons bouncing off at various 
angles was noted.  
What were they expecting to see in this 
experiment?
They were expecting the electron beam would be 
diffusely reflected and a smooth distribution of 
intensity would be observed 
as a function of angle theta.
What they saw was something different !
Strong maxima in the intensity, very large number 
of reflected electrons, were seen at some 
particular angles.
A graph between intensity and scattering angle 
theta, at a specific angle there was high intensity 
which meant large concentration of electrons.
Davisson and Germer 
observed a similarity between the results that 
they got and x-ray diffraction experiments that 
had been done earlier.
They concluded that the electron beam was 
being diffracted. 
and this was a direct experimental evidence of 
de Broglie’s wave hypothesis. 
Say these are the incident waves and these are 
the scattered waves. Assuming an accelerating 
voltage of 54V which means the
electrons have been accelerated through a pd of 
54V.
now for these electrons the maxima was 
observed at 50 degrees.
This results from constructive interference 
between electron waves scattered from various 
atoms in the surface layer of the crystal. These 
rows of atoms act like a
 reflecting diffraction grating.
The condition for  maximum reflection is given by 
d sin theta = m x lambda 
where m = 1,2,3…, 
where d is the distance between adjacent rows 
of atoms in the surface plane. 
because most of the electrons will be scattered 
by atoms present on the surface of the crystal.
For an accelerating voltage of 1400V the maxima 
was observed at 34 degrees. 
For an accelerating voltage of 1400V the maxima 
was observed at 34 degrees. 
The angular positions of the maxima depended on 
the accelerating voltage used to produce the 
electron beam. 
The angular positions of the maxima depended on 
the accelerating voltage used to produce the 
electron beam. 
now let us see the formula that we use
the de Broglie wavelength of a particle is given 
by lambda = h/p 
Let us express the wavelength of the particle in 
terms of its kinetic energy. 
Let us express the wavelength of the particle in 
terms of its kinetic energy. 
An electron, that is at rest is accelerated from 
point a to point b through a pd of Vb- Va= Vba. 
An electron, that is at rest is accelerated from 
point a to point b through a pd of Vb- Va= Vba. 
and it acquires a velocity v, in this case the work 
The work done on the electron eVba is equal to 
the kinetic energy of the electron.  
