1. The problem statement, all variables and given/known data Find the initial wavelength of a photon that loses half its energy when it Compton-scatters from an electron and emerges at 90∘ to its initial direction of motion. 3. The attempt at a solution Ei = Ef +KE Ei = 0.5Ei + KE Using compton's formula for scattering of photon: Δλ = λf - λi = 2.42444x10-12 m λi = λf - 2.42444x10-12 m pretty much stuck. If I know the initia energy, find the initial wavelength would be a breeze but I don't have that information. Or if I know the numerical value of KE, I can by KE = 0.5Ei, find initial wavelength.