Short Answer
Lc = c/Δν — path difference over which fringes remain visible 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.
L_c = c/Δν characterises temporal coherence — fringes are visible only when path difference < L_c.
Why This Answer Is Correct
This is a Hard-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. The distance light travels in one period
- B. Lc = c/Δν — path difference over which fringes remain visible
- C. Lc = λ/NA
- D. Physical length of the source
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: Hard
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.