An alpha particle and a proton are accelerated from rest through the same potential difference V. Find the ratio of de-Broglie wavelengths associated with them.
The de-Broglie wavelength associated witha proton having charge e and accelerated through a potential difference V is given by
λ = h /( 2mev)1/2 ......... (i)
suppose that α - particle posses the de-Broglie wavelength λ' , when accelerated through the same V. if e' amd m' are the values of charge and mass of the α -particle , then
λ' = h/(2m'e' V)1/2 ......... (ii)
dividing eqn (i) by (ii) , we have
λ / λ' = h / (2meV)1/2 x (2m'e' V)1/2 / h
= (m' e' / m e)1/2
as u know , mass of alpha particle is 4 times the proton and charge is 2 times greater,
m' = 4m ; e' = 2e
therefore
λ / λ ' = (4m x 2e / m e)1/2
= (8)1/2