Question

If the source is moving away from the observer, then the apparent frequency ___________.

• A. will increase
• B. will remain the same
• C. will be zero
• D. will decrease

Answer 1

The correct option is D will decrease

Doppler effect states that when the source of sound and the observer are relatively approaching each other, the apparent frequency of the sound increases and decreases when they are seperating.

Applying this logic, if the source is moving away from the observer, then the apparent frequency will decrease.

$f_{apparent}=f_{real}\times \left(\frac{{}_V{\pm }_{V_O}}{{}_V{\mp }_{V_S}}\right)$------------------------------$\left(I\right)$

Here V is the speed of sound, $V_O$ is of observer, $V_S$ of source

Now you will wonder as when it'll be + and when -.

For this you must understand the logic very well.

When the source of sound and the observer are relatively approaching each other, the apparent frequency of the sound increases and decreases when they are seperating.

Numerator should be more and denominator should be less when apparent frequency is more and vice-versa.

This is how you can remember the formula.