A proton an an `alpha-`particle, moving with the same velocity, enter a uniform magnetic field, acting normal to the plane of their motion. The ratio of the radii of the circular paths descirbed by the proton and `alpha`-particle is

Explanation

`r_p = (mv)/(qB) and r_(prop) = (4mv)/((2q)B) = 2r_(prop) rArr (r_(p))/(r_(prop) )=1/2`.