# Change in Frequency/Wavelength in Doppler Effect

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= ff - fi or Δλ=λf - λi
* Source approaching: frequency increase , hence wavelength gets shorter
* Source moving away : frequency decrease , hence wavelength gets longer

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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.

• 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= ff - fi or Δλ=λf - λi
* 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.
https://s-media-cache-ak0.pinimg.com/originals/fa/4d/02/fa4d027ccc0ebeca28acd83c84aa2861.jpg

• Samurai44