An alpha-particle makes an elastic head-on collision with a proton initially at rest. Ratio of de-Broglie wavelength associated with alpha-particle and proton after collision will be

Let initial speed of α is v0 and final  speeds of α and proton are vα and vp .  Then, momentum conservation gives   4mpv0=4mpvα+mpvp    ∵mα=4mp  Also, e=1 ; elastic collision  ⇒  v0-0=vp-vα  Elimination of v0 gives,  vα=38vp  Now, λαλp=hmαvα×mpvph=mpmα·VpVα=14×83=23