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Doppler Effect (xkcd.com)

submitted 5 months ago by [email protected] to c/comicstrips

17 comments fedilink hide all child comments

The Doppler effect is a mysterious wavelength-shifting phenomenon which seems to primarily affect sirens, which is why the 🚨 emoji is red.

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[–] disguy_ovahea -3 points 5 months ago* (last edited 5 months ago) (2 children)

Red is the fastest color in the visible light spectrum, and violet is the slowest. A cool use of this phenomenon is lightning strike camera triggers. It senses the invisible infra-red light, which hits the sensor before visible light, and takes the picture just in time to capture the lightning strike.

Edit: This is only true when traveling through atmosphere. As pointed out below, light travels at uniform speed through a vacuum.

Also, the comic is referring to the Doppler Effect, which is caused by an object in motion, having nothing to do with light spectrum speed through atmosphere. My bad.

[–] ccunning 21 points 5 months ago (1 children)

It’s also why all red cars automatically get +5HP…

[–] [email protected] 8 points 5 months ago

Also

DA RED WUNZ GO FASTA!

[–] subtext 17 points 5 months ago (2 children)

This does not seem correct at all… as the comic states, it’s caused by the Doppler effect, not because red is the “fastest” light wavelength—it’s light it all travels at the same speed in a vacuum.

https://en.wikipedia.org/wiki/Redshift

I don’t know about lightning photography, I suppose it’s possible that different wavelengths have different speeds in air, so I can’t comment on that.

[–] kernelle 11 points 5 months ago

Yeah I'm no physicist but that ticked me off, the speed of light is the same for any wavelength. As for redshift:

a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light).

Speed of light isn't a factor in this, also when galaxy's move towards us (like the Andromeda galaxy) it is blueshifted, proving it's not the light that matters, but rather the direction of movement of the source. Proving the doppler effect.

[–] [email protected] 2 points 5 months ago

Could this be because space isn't a true vacuum? There's pockets of gas and crap, which could affect the speed of the light as it travels to us?