Question

Imagine Earth receives a radio wave signal from space that is observed to be increasing in frequency. Assume the source is producing a constant frequency and intensity wave. (a) If we do not know the original frequency of the source, can we determine if the object is drawing closer or moving farther away from us? Explain your reasoning. (b) What other characteristic of the wave could we use to determine if the source is approaching or receding?

Answer 1

a) If the observer frequency is increasing, then the object is drawing closer to us.

Suppose a source emitting light beam at a certain interval is drawing towards us. As it comes closer to us, the distance between the source keeps decreasing and hence the time interval keeps decreasing. This means the frequency increases.

b) We can also figure out if the wavelength of the radio wave is decreasing or increasing. Since the velocity of electromagnetic waves remains constant, and wavelength is inversely proportional to wavelength, an increase in frequency means, the wavelength has decreased.

This phenomenon is called blueshift.

So if the waelength is decreasing, the source is approaching and if the wavelength is increasing, then the source is receding.