One-line rule for each key formula:

1. $\beta = \lambda D/d$ — "fringe width = lambda D over d"
2. Bright: $\Delta = n\lambda$ — "whole wavelengths"
3. Dark: $\Delta = (2n-1)\lambda/2$ — "half-odd wavelengths"
4. $I = 4I_0\cos^2(\phi/2)$ — intensity in YDSE
5. $\phi = (2\pi/\lambda)\Delta$ — phase from path difference
6. Single slit central max width $= 2\lambda D/a$ — "twice the secondary"
7. Diffraction minimum: $a\sin\theta = n\lambda$
8. Brewster: $\tan\theta_p = n$ — "tan = n"
9. $\theta_p + \theta_r = 90°$ — "perpendicular at Brewster"
10. Malus: $I = I_0\cos^2\theta$ — "cosine squared"
11. Three polaroids (45°): $I = I_0/8$
12. In medium: $\lambda' = \lambda/n$, $\beta' = \beta/n$

What NEET tests most: $\beta = \lambda D/d$ and its variations — this is #1 most tested formula in wave optics.

Last 5-minute revision:

• YDSE: $\beta = \lambda D/d$; bright at $n\beta$; dark at $(n-\frac{1}{2})\beta$
• Intensity: $I_\text{max} = 4I_0$, $I_\text{min} = 0$
• Brewster: $\tan\theta_p = n$; reflected ⊥ refracted
• Malus: $I = I_0\cos^2\theta$; three polaroids → $I_0/8$
• Single slit: central width $= 2\lambda D/a$; minima at $a\sin\theta = n\lambda$