Most Frequently Tested Topics (NEET 2015–2024):

1. Fringe width formula and variations (NEET 2018, 2020, 2022):

   • Direct calculation of $\beta = \lambda D/d$
   • Effect of changing $D$, $d$, $\lambda$, or medium
   • Immersing YDSE in liquid → fringe width changes

2. Polarization — Brewster and Malus (NEET 2021, 2024):

   • Calculating Brewster's angle given $n$
   • Three-polaroid problem with middle at known angle
   • Transmitted intensity through crossed polaroids

3. Conceptual MCQs — Coherence and White Light (NEET 2019, 2023):

   • Why two independent sources can't interfere
   • Central fringe color in white light
   • Effect on fringe pattern when one slit is blocked

4. Single slit diffraction (NEET 2016, 2020):

   • Central maximum width
   • Comparison with YDSE fringe width
   • First minimum position

High-Yield Formula for NEET: $\boxed{\beta = \frac{\lambda D}{d}}$ This single formula, with all its variations, accounts for approximately 1–2 NEET questions each year.