When light is shone from an approaching object towards a stationary observer, the light is blue-shifted. What exactly is the amount of this change in frequency? So if the wave that leaves a radar gun is, say, 500 THz, when it is travelling 100km/h, what will it be when observed by the stationary observer? This is NOT a homework question and I am not a student trying to do homework so please don't assume I'm being 'lazy' and not doing my homework myself. What I'm trying to figure out is if the change in frequency observed is linear compared to the speed, or if it is relativistic and a small change in frequency compared to the speed. Thanks.