A wavefront is the locus of points in the same phase, and Huygens' principle — every wavefront point acts as a new secondary source — provides the geometric basis for deriving Snell's law. In Young's double slit experiment, two coherent slits separated by $d$ at distance $D$ from a screen produce equally spaced fringes of width $\beta = \lambda D/d$. Bright fringes form where the path difference equals a whole number of wavelengths ($\Delta = n\lambda$), and dark fringes form at odd half-wavelength differences ($\Delta = (2n-1)\lambda/2$). The intensity at any point is $I = 4I_0\cos^2(\phi/2)$, giving $I_{\max} = 4I_0$ and $I_{\min} = 0$ for identical slits. Coherent sources must share the same frequency and a constant phase difference; two independent bulbs or even two separate lasers cannot sustain a visible fringe pattern. Single slit diffraction produces a central maximum of width $2\lambda D/a$, which is twice as wide as secondary maxima, and minima at $a\sin\theta = n\lambda$. Polarization proves light is a transverse wave — its electric field oscillates perpendicular to the propagation direction and can be restricted to one plane by a polaroid. Brewster's law states $\tan\theta_p = n$; at this angle the reflected ray is completely plane-polarized and perpendicular to the refracted ray. Malus's law gives the intensity of polarized light through an analyser: $I = I_0\cos^2\theta$, yielding zero through crossed polaroids and $I_0/8$ when a third polaroid is inserted at $45°$. In any medium of refractive index $n$, the wavelength shortens to $\lambda/n$ and the fringe width shrinks proportionally to $\beta/n$.