Read before you solve · 3 min
Read YDSE: Convert Position into Path Difference
Fringes are counted through path difference, not by visual spacing alone. First convert screen position to δ = d y/D.
NSEP · Ray Optics · 1 linked question · 100% coverage
Know this first
Young's Double-Slit Fringe Counting is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Study deeper: NCERT Physics · Ray Optics
Start with the condition
Bright fringes satisfy
dark fringes satisfy
δ = (m plus one-half) λ
Locate the stated bright fringe order, then count only dark orders strictly between it and the centre.
Build the governing relation
Keep the centre as
m = 0 and distinguish whether an endpoint is bright or dark before counting
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
OlympIQ trap
Rounding the bright-fringe order before checking the exact condition can add or remove a dark fringe.
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