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

Do you mean the colors applied to the images rendered by the ultrasound software? Because if so, that’s just the convention the manufacturer adopted when creating the machine.

Ultrasonic waves aren’t electromagnetic waves so the concepts of “red” and “blue” don’t apply to the Doppler effect in that medium.