Fringe width: $\beta = \lambda D/d$. Fringe width is proportional to $\lambda$ and $D$, inversely proportional to $d$. In a medium of refractive index $n$, $\beta$ decreases by factor $n$ because $\lambda \to \lambda/n$.

YDSE conditions: Bright fringe at $\Delta = n\lambda$; dark fringe at $\Delta = (2n-1)\lambda/2$. Intensity: $I = 4I_0\cos^2(\phi/2)$; maximum $= 4I_0$, minimum $= 0$ (equal slits).

Single slit central maximum is exactly twice as wide as each secondary maximum: width $= 2\lambda D/a$ vs $\lambda D/a$. Minima at $a\sin\theta = n\lambda$.

Brewster's law: $\tan\theta_p = n$. Reflected light is completely polarized; $\theta_p + \theta_r = 90°$.

Malus's law: $I = I_0\cos^2\theta$. Unpolarized → first polaroid → $I_0/2$. Three polaroids (crossed with middle at $45°$) → $I_0/8$.