1. The problem statement, all variables and given/known data 2. Relevant equations 2. (b) I didn't understand how can I find the app. wavelength 3. The attempt at a solution I tried : the electron move over λu with velocity β so the wavelength is λ≈λu /β but its not the right answer. my lecturer told me I should consider also the dopler I only know that the answer is : λ≈λu /(2*γ2) γ= gamma THANK YOU!