eli5: Why does blue shift in astronomy equal an approaching object and blue color in doppler ultrasound equal movement away?

Question

I’m in a class that is discussing the operation of diagnostic ultrasound and I was confused by the colorization.

In: Physics

Answer 1

The terms come from visible-light astronomy.

In visible light, the longest visible wavelength is “red”, while the shortest is “blue”. Thus, stretching visible light makes it look redder, and squishing it makes it look bluer.

And then, this “red/blue” direction structure serves as a good metaphor for conceptualizing other types of wave.