Diagnostic ultrasound uses high frequency (typically 1-20 MHz) sound waves. Pulses of high frequency sound waves are generated by an ultrasonic transducer that is placed on the patient’s skin. While most of the sound waves are scattered and absorbed within the patient, some are reflected by internal body structures (at boundaries between two dissimilar materials). When these reflected sound pulses return to the surface, they are received by the transducer and then amplified and processed to form an image.

Visible light is one form of something that we call electromagnetic radiation. Radio, microwaves, infrared, ultraviolet, x-rays, and gamma rays are other forms of electromagnetic radiation. They are all exactly the same as visible light except that they have different wavelengths. That is the only difference. (The fact that human eyes can only see a narrow range of these wavelengths does not make them different). Therefore, all electromagnetic radiation experiences the Doppler Effect. It is manifested as a shift in the wavelength, just like visible light.

Could you explain,please.

By their very nature, one half of a helicopter’s rotating blades will look like they’re coming faster towards you than the helicopter, and one half will look like they’re going away from you faster than the helicopter. That’s just the nature of the rotating blades.

I don’t know much about stealth helicopters. I suppose that if the fuselage was really “stealthy” and didn’t have much of a return signal on radar then you might be left with just the signal from the main roter blades, and that might give you some kind of a weird, “it’s coming _and_ it’s going,” doppler return. Just speculating.

Stars can show a similarly confusing doppler shift because they’re being tugged back and forth by the gravity of planets in orbit around them. In fact, looking for stars with spectra that appear to be shifted both towards the red and the blue (the star looks like it’s simultaneously approaching and receding) is one of the most commonly used ways astronomers use to detect exoplanets.

I’ve been really curious about something that I heard on the news when the USA went into Pakistan to get Osama Bin Laden.

I heard that the fancy helicopter that crashed was fitted with technology that made it sound like it was getting further away even though it was getting closer.

My first reaction was that surely this is impossible, because of the doppler effect.

Do you have any thoughts on that? It’s got to be rubbish surely!

Cheers
Leigh