1. The problem statement, all variables and given/known data A photon scatters in the backward direction ([tex]\theta[/tex]= 180) from a free proton that is initially at rest. What must the wavelength of the incident photon be if it is to undergo a 10.0% change in wavelength as a result of the scattering? 2. Relevant equations [tex]\lambda[/tex]'-[tex]\lambda[/tex] = (h/mc)(1-cos([tex]\theta[/tex])) where the left side is the difference between scattered and incidence wavelengths. 3. The attempt at a solution This seemed like a pretty straightforward problem. Since the photon undergoes a 10% change in wavelength, 1.1[tex]\lambda[/tex] = [tex]\lambda[/tex]'. Therefore .1[tex]\lambda[/tex] = (h/mc)(1-cos([tex]\theta[/tex])). Multiply by 10 and evaluate the cosine, and you get [tex]\lambda[/tex] = 20h/mc. However, when I substitute values into this and evaluate it I get the wrong answer. I have absolutely no clue what I am doing wrong here. This shouldn't be a difficult problem, but for some reason I am not getting the correct answer. Help would be appreciated. Thanks.