Radius of a charged particle in a magnetic field is given by : R=mVqB
∴ momentum = p=mV=RqB
It is given that radius of alpha particle is half of electron.
We know that the charge of an alpha particle is twice of that of an electron.
Therefore momentum remains same for both electron and alpha particle .
de Broglie wavelength = hp
Since momentum is same, ratio of wavelengths is 1 .