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## Homework Statement

A photon with wavelength 0.1385 nm scatters from an electron that is initially at rest. What must be the angle between the direction of propagation of the incident and scattered photons if the speed of the electron immediately after the collision is 9.30×106 m/s?

## Homework Equations

$$\lambda' - \lambda = h/mc (1-\cos\phi)$$

## The Attempt at a Solution

I have $$\lambda' = 0.1385 nm$$ but I'm unsure as to how to get the original wavelength so that I can use the equation stated above. I think I need to use conservation of momentum $$\overrightarrow{Pe} = \overrightarrow{p} - \overrightarrow{p'}$$ but I'm not really sure how to apply it.