# Doppler Effect

## Homework Statement

Hi guys can someone clarify something in my text. I have included a screenshot, and marked the area in question.
Shouldnt it be that as the point source approaches you the intensity AND frequency increase? (see the pitfall prevention)

[PLAIN]http://img10.imageshack.us/img10/7357/27411329.png [Broken]

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Doc Al
Mentor
Shouldnt it be that as the point source approaches you the intensity AND frequency increase? (see the pitfall prevention)
No. Don't confuse this with a comparison between a stationary source and a moving source. In that case, the observed frequency with a moving source will be higher than for a stationary source. The point they are making here is that the observed frequency doesn't change as the source approaches (until it passes you).

ok i think i see where i made the mistake:

For a stationary source:
as the observer moves toward the source with velocity v, the the frequency obsered would increase as well as the intesity.

as the observer moves away, the frequency would decrease as well as intensity.

for a stationary observer:
as the source moves toward the observer frequency is the same but intensity increases
(and observed wavelength decreses)

if the source moves away, the observe frequency, intensity, and wave length would decrease.

thanks for your time Doc al

I think im making several mistakes, im not distinguishing between the observed frequency and the actual frequency. I will reread the section.

[PLAIN]http://img35.imageshack.us/img35/2637/40068221.png [Broken]

Last edited by a moderator:
Doc Al
Mentor
ok i think i see where i made the mistake:

For a stationary source:
as the observer moves toward the source with velocity v, the the frequency obsered would increase as well as the intesity.

as the observer moves away, the frequency would decrease as well as intensity.
No, I think you're still missing the point of that "pitfall prevention" tip.

If you move towards a stationary source, the observed frequency will be higher than the source frequency. But as you move towards the source, the observed frequency doesn't change. It's higher than the source frequency, but remains constant as you approach the source. (Some folks mistakenly think that the closer you get to the source, the higher the frequency. That's the misconception that they are trying to point out.)

for a stationary observer:
as the source moves toward the observer frequency is the same but intensity increases
(and observed wavelength decreses)

if the source moves away, the observe frequency, intensity, and wave length would decrease.
The same points made above would apply here. If the source approaches you, then the observed frequency will be higher than the source frequency. But it doesn't change as the source gets closer.

i get it thank you! the observed frequency is higher then the source frequency BUT the frequency you observe while stationary is constant until it passes you.
Frequency you observe is dependant of the speed of the source.
i wonder what would happen if the source moved faster then the wave...

Doc Al
Mentor
Good!