Physics Waves MDCAT 2015
PMDC Verified Question 113 of 125
An observer moves with velocity '\( u_o \)' toward a stationary source, then the number waves received in one second is
A
\( f' = f\left(\frac{v}{v+u_o}\right) \)
B
\( f' = f\left(\frac{v+u_o}{v}\right) \)
C
\( f' = f\left(\frac{v}{v-u_o}\right) \)
D
\( f' = f\left(\frac{v-u_o}{v}\right) \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: \( f' = f\left(\frac{v+u_o}{v}\right) \)
Concept:

"Number of waves received in one second" is the definition of apparent frequency.

Solution:

  • When an observer moves towards a stationary source, they "intercept" more wave fronts per second than if they were standing still.


  • The relative speed of the waves relative to the observer becomes \( v + u_o \).


  • Using \( v = f\lambda \), the new frequency is \( f' = \frac{\text{relative speed}}{\lambda} = \frac{v + u_o}{\lambda} \).


  • Since original \( \lambda = \frac{v}{f} \), we substitute to get \( f' = \left(\frac{v + u_o}{v}\right) f \).


Why other options are incorrect:

Option D represents an observer moving away. Options A and C are incorrect mathematical formats for a moving observer (those formats are for a moving source).

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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