Change in Frequency/Wavelength in Doppler Effect

[Samurai44]

Greetings ,
Many Doppler effect questions require to find the final Frequency/Wavelength. so after using the equation :
Δλ / λ = v / c or Δf / f = v / c
How can I know if the change in Frequency/Wavelength is positive or negative to calculate the final frequency or wavelength. because sometimes they add the change to initial but in others they subtract .

i know this's all related to :
* Δf= f_f - f_i or Δλ=λ_f - λ_i
* Source approaching: frequency increase , hence wavelength gets shorter
* Source moving away : frequency decrease , hence wavelength gets longer

 

[theodoros.mihos]

One way to imagine this fact is to thinking about the distance. If distance get shorter then λ get shorter. Source frequency does not change but for the adapter it likes to be larger.

 

[nasu]

Greetings ,
Many Doppler effect questions require to find the final Frequency/Wavelength. so after using the equation :
Δλ / λ = v / c or Δf / f = v / c
How can I know if the change in Frequency/Wavelength is positive or negative to calculate the final frequency or wavelength. because sometimes they add the change to initial but in others they subtract .

i know this's all related to :
* Δf= f_f - f_i or Δλ=λ_f - λ_i
* Source approaching: frequency increase , hence wavelength gets shorter
* Source moving away : frequency decrease , hence wavelength gets longer

It will depend on the situation. You need to understand various cases (there are 4 cases for sound) and pick-up the right sign.
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