1. The problem statement, all variables and given/known data Suppose a frame S' is related to S by a boost in the y direction by v. Imagine a wall is at rest in the S' frame along the line y'=-x'. Consider a particle moving in the x'y' plane that strikes the surface and is reflected by the usual law of reflection θ'i=θ'r. Find the relation between the incident and reflected angles in the S frame as well as the initial and final velocity of the particle in the S frame. 2. Relevant equations Velocity addition, lorentz contraction 3. The attempt at a solution I assumed for simplicity that, although the wall is in S' and moving, I would always be able to redifine the S origin so that the wall passes through the origin in both frames. Now the wall itself will be contracted in the y direction, so the equation of the wall in the S frame is y/β=-x (I'm using beta for the lorentz factor because gamma looks too similar to y). To find the angle of reflection and incidence in the S' frame I need a normal vector to the wall, which is easily obtained to be 1/√2*(i'+j'). Now the velocity of the particle in the S' frame is u'=uxi'+uyj' After striking the wall and being reflected, the particle's velocity will be reflected and reversed about the normal to the wall so that after contact the new velocity will be u=-uyi'-uxj'. Now the incident and reflected angles can be calculated (and are equal in this frame) by cosθ=n*u/|u| where n is the unit normal to the wall 1/√2*(i'+j') To calculate the angles in the S frame we have to transform the initial and final velocities of the particles and then dot them with the NEW normal to the wall which can be obtained from y/β=-x, remembering that I'm using beta for the lorentz factor instead of gamma. The normal to the wall in this frame can be considered by a point on the perpendicular line y=x/β considering when x=1 y=1/β the normalized unit vector along this line is then (i+1/βj)/((1/β)2+1)1/2 Once I transform the above velocities via the addition formula and dot it with this normal vector, will I arrive at the correct answer? Is there an easier way to arrive at this result if it is correct? Forgive me for not typing the entire solution, but I feel like this is already in the realm of tldr.