The Doppler Effect
The Doppler Effect, named after Austrian physicist Christian Doppler, refers to the change in frequency of a wave for an observer moving relative to the wave’s source. The most common everyday instance of this is for a siren or horn passing an observer in a car - as the siren approaches, passes and continues, the received frequency changes multiple times, resulting in a recognizable change. As the siren approaches, the frequency is higher, and the pitch appears higher - and as it passes and moves away the frequency recedes and lowers.
As the source of the waves moves towards the observer, each successive crest is emitted closer to the observer than it was previously. Thus, the frequency (number of waves per second) increases because each wave is closer to the observer than the previous one. The opposite effect happens as the waves move past the observer, it takes longer for each to reach the observer - and the frequency drops.
The doppler effect is also readily present in astronomy, as it was the signal that tipped off Edwin Hubble to note that the universe must be expanding. During his studies at the Mount Wilson Observatory, he noticed that galaxies experienced a ‘red-shift,’ and the shift increased as the galaxies got further away. This red-shift is caused by the Doppler Effect, except in this case the waves are not sound waves like in the siren example but electromagnetic (light) waves. Since the galaxies are moving away from us, the frequency of the light is decreasing - and so the light moves towards the red side of the spectrum and appears to be this color.
As the sticker suggests, as moving objects move increasingly closer to an observer at the speed of light - the waves are ‘blue-shifted,’ and the object appears blue. Although the driver of the car could never reach the speed of light, it’s still a damn good sticker.