1. The problem statement, all variables and given/known data Why does a source moving perpendicular to the line between it and the observer produce no doppler effect? 2. Relevant equations 3. The attempt at a solution I have heard the the radial velocity of the source relative to the observer must be zero but i dont understand why, if the distance between the source and the observer is increasing or decreasing , shouldn't there be a doppler effect since the wave length is changing?