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


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

In: Physics

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.

A blueshift corresponds to an increase in frequency of returning waves. Returning waves are ‘bluer’ as they are higher frequency.

When an ultrasound produces an image, it is because a computer takes those sound waves and records their strength/frequency. This is two numbers, but has no intrinsic color. The computer chooses to translate these two numbers into color and brightness which are then displayed to you. What color represents what frequency shift is totally arbitrary.

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.

The operator has control over the color scale on an ultrasound machine and can invert as needed. This is done mostly due to convention and for consistency – arteries are displayed as red, veins as blue.