Concept:The Doppler Effect predicts a change in observed frequency when there is relative motion. An approaching source compresses the wave fronts in the direction of travel.
Formula:$$ f_o = \left( \frac{v}{v - u_s} \right) f $$
Solution:- Because the source is moving towards the observer, the waves are "crowded" together. This physically shortens the wavelength.
- A shorter apparent wavelength directly results in a higher apparent frequency.
- To yield a fraction greater than 1, the denominator must be decreased. Therefore, we subtract the source velocity (\( u_s \)) from the wave velocity (\( v \)) in the denominator.
Why other options are incorrect:Option B represents a source moving away. Options C and D wrongly place the variables (using observer velocity \( u_o \) in the denominator instead of the numerator).
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