Doppler Effect equation

[millepora]

Examkrackers gives a more simplified equation for wavelength and frequency.
^f/f=v/c. The explanation really doesn't give a good explanation for what I should plug "c" and what I should plug into "v" when given a problem. Can anyone help me better understand using this equation?

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[Bernoull]

Examkrackers gives a more simplified equation for wavelength and frequency.
^f/f=v/c. The explanation really doesn't give a good explanation for what I should plug "c" and what I should plug into "v" when given a problem. Can anyone help me better understand using this equation?

df/fs = v/c df=change in freq, fs= source freq, v= relative velocity b/t source n observer, c =source velocity.

When you find relative velocity just remember ur adding/substracting vectors so pay attention to directions of both!!

Goodluck

 

[MBHockey]

I think it's more likely you'll have to understand this from a conceptual perspective on the real test (although I do agree that doing problems will help you solidify your understanding.) If they want you to use the formula they'll likely provide it.

Just understand that an increase in speed of the wave results in a higher frequency. Then you don't have to worry about knowing which variable stands for what in the equation.

If you are standing still and I run at you, screaming at the top of my lungs, the pitch that you perceive will be higher than what is actually being generated by my vocal chords, because the wave is coming faster since I am running towards you. Similarly, if i am screaming at you but running backwards, the pitch you perceive will be lower.

If you understand this scenario you should be able to apply that understanding to any combination of relative velocities between source and observer.

Why does the frequency get higher as I run toward you? It's because as I am running at you, I'm pushing the wave towards you and more peaks hit your eardrum per second than if I was just standing still. This, by definition, is an increase in frequency.

 

[bunnity]

The equation the OP posted is the doppler equation for light / electromagnetic waves. So "c" is always the speed of light, while "v" is the relative speed of the source and observer (so if the source is traveling 3 m/s east and the observer is traveling 2 m/s east v=1 m/s).

The doppler equation for sound is different, and does not use the variable "c." So while the example MBHockey posted is good conceptually and makes sense for sound, remember light and sound have different looking doppler equations.

 

[sv3]

The equation the OP posted is the doppler equation for light / electromagnetic waves. So "c" is always the speed of light, while "v" is the relative speed of the source and observer (so if the source is traveling 3 m/s east and the observer is traveling 2 m/s east v=1 m/s).

The doppler equation for sound is different, and does not use the variable "c." So while the example MBHockey posted is good conceptually and makes sense for sound, remember light and sound have different looking doppler equations.

i thought light and sound use the same equation? Just the speed of sound vs the speed of light changes in the equation but the structure is the same. I'm talking about the equation form:
Freq detected = freq source x (speed of sound or light+/-detector speed divided by speed of sound or light +/- source speed)

is this wrong?

thanks in advance

 

[bunnity]

I don't have my book anymore but this is courtesy of wikipedia:

For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in special relativity, only the relative difference in velocity between the observer and the source needs to be considered.

So they are different - it's the same idea that observed frequency increases when things are moving toward each other and decreases when they are moving away, but my book definitely had two different equations. Wikipedia has them as two different equations as well.

 

[Bernoull]

i thought light and sound use the same equation? Just the speed of sound vs the speed of light changes in the equation but the structure is the same. I'm talking about the equation form:
Freq detected = freq source x (speed of sound or light+/-detector speed divided by speed of sound or light +/- source speed)

is this wrong?

thanks in advance

For MCAT purposes the equation is the same, EK even states the c in the formula is NOT necessarily speed of light. c is just wave source velocity (be it sound or electromagnetic), there are some differences due to relativistic effects in electromagnetic doppler shift but this is beyond the scope of the MCAT, if it's tested it'll be explained in a passage and relevant eqs provided. FOr the MCAT Doppler shift is simply:

fs=fo((v +/- vs)/(v +/- vo))

df/fs = v/vs df=change in freq, fs= source freq, v= relative velocity b/t source n observer, vs =source velocity.

So u got it right sv3.. FYI: The formula u sued and the one i used are algebraically equivalent..

 

[sv3]

For MCAT purposes the equation is the same, EK even states the c in the formula is NOT necessarily speed of light. c is just wave source velocity (be it sound or electromagnetic), there are some differences due to relativistic effects in electromagnetic doppler shift but this is beyond the scope of the MCAT, if it's tested it'll be explained in a passage and relevant eqs provided. FOr the MCAT Doppler shift is simply:

fs=fo((v +/- vs)/(v +/- vo))

df/fs = v/vs df=change in freq, fs= source freq, v= relative velocity b/t source n observer, vs =source velocity.

So u got it right sv3.. FYI: The formula u sued and the one i used are algebraically equivalent..

thanks bud. Felt pretty good about my understanding so that shook me a little.....glad to see im alright after all. Was just reviewing physics/chem/orgo before i hit bio....sure hope 3-3.5 wks is enough!

thanks
steve

 

[RevDM]

This is the easiest way I've learned the equation.

Use the TOP symbols (+ and -, respectively) when they are moving toward, and the bottom ( - +) when moving away

f = fo (v ± vo)/(v -/+ vs)