An x-ray photon is scattered from a free electron (mass m) at rest. The wavelength of the scattered photon is λ′, and the final speed of the struck electron is v.

What was the initial wavelength λ of the photon? Express your answer in terms of λ′, v, m, and the appropriate constants. For the wavelength of the scattered photon λ′ use the notation λf. (Hint: Use the relativistic expression for the electron kinetic energy.