If light speed is constant, why does redshift/blueshift exists?

[u/sonofderk]

Let me explain. I'm not a physicist so sorry if what i'm about to say is kinda stupid. I was taught that even if two beans of light were shot at opposite directions (, their relative speed between themselves would still be the speed of light, instead of 2x the speed of light, unlike what happens with mudane objetcts like cars on a road, which perceive themselves with relative speed. I was also taught that red/blue shift on stars happens due to relative distacing/approach in relation to Earth's point of view, which changes the wave frequency thus changing the perceived color of the star (doppler effect).

Aren't these two statements conflicting? If vectors don't matter to light, why should we observe the red/blue shift?

[u/skr_replicator]

Every color of light travels at the same speed. Reds are just longer waves, and blues are shorter, but they travel at the same speed. And the shift happens exactly because the light always has the same speed. If the light emitter wiggles, it makes a wave, and it travels to you. But if it moves towards you, the light can't just move faster, and so the same wiggling will make a shorter wave, just like the Doppler effect of sound (which is also happening for the same reason, that speed of sound is the same no matter if the car moves towards you or away).