X-Rays are produced by electrons accelerated through an electric potential difference of 50kV. An electron makes three collisions in the target coming to rest and loses half of its remaining kinetic energy in each of the first two collisions. Determine the wavelength of the remaining photons.(Neglecting the recoil of the heavy target atoms)

Its conservation of energy . What you can do is , equate loss in energy to hf

 

E = hf

 

f = frequency of x rays