Short Answer
f' = f·c/(c−v_s) is the best answer.
Wave and optics questions test how frequency, wavelength, phase, interference, and geometry fit together. Start with the physical picture before choosing the equation.
f' = f·c/(c − v_s): as source approaches, denominator decreases, frequency increases.
Why This Answer Is Correct
This is a Medium-level question in Waves & Optics. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
If a waves question feels messy, sketch the geometry or phase relationship first, then return to the algebra.
Choices At A Glance
- A. f' = f(c+v_s)/c
- B. f' = f·c/(c−v_s)
- C. f' = f−v_s/c
- D. f' = fc/(c+v_s)
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Topic Snapshot
Topic: Waves & Optics
Difficulty: Medium
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.