Subtopic 1: Wavefronts and Huygens' Principle

A wavefront is a surface of constant phase. Types: spherical (point source), cylindrical (line source), plane (distant source). Huygens' principle: each wavefront point emits secondary spherical wavelets; the next wavefront is the forward envelope. Application: derives Snell's law geometrically — when a wavefront enters a denser medium, it slows down and tilts, reproducing $n_1\sin\theta_1 = n_2\sin\theta_2$. NEET tests this conceptually: "which type of wavefront does a distant star produce?" (Answer: plane wavefront).

Subtopic 2: Young's Double Slit Experiment

This is the highest-weightage subtopic. Core formula chain:

• Path difference: $\Delta = dy/D$
• Bright fringe: $\Delta = n\lambda \Rightarrow y_n = n\lambda D/d$
• Dark fringe: $\Delta = (2n-1)\lambda/2 \Rightarrow y_n = (2n-1)\lambda D/(2d)$
• Fringe width: $\beta = \lambda D/d$
• Effect of medium: $\beta_{\text{medium}} = \beta_{\text{air}}/n$
• Intensity: $I = 4I_0\cos^2(\phi/2)$

NEET tests: fringe width change when $D$, $d$, or $\lambda$ is altered; fringe pattern in white light (central fringe is white); YDSE in a liquid (fringe width decreases); and why two separate laser beams cannot produce sustained fringes (not coherent).

Subtopic 3: Single Slit Diffraction

One aperture of width $a$. The central maximum is a broad bright band centred at $\theta = 0$, flanked by alternating dark and secondary bright bands. Minima at $a\sin\theta = n\lambda$; secondary maxima at $a\sin\theta = (2n+1)\lambda/2$. Angular half-width of central maximum = $\lambda/a$. Linear full width of central maximum = $2\lambda D/a$. The central maximum is twice as wide as each secondary maximum — this comparison with YDSE (where all fringes are equal) is a standard NEET discrimination question.

Subtopic 4: Coherence

Two sources are coherent if they emit light of the same frequency with a constant phase difference. A single source illuminating two slits (as in YDSE) guarantees coherence. Two independent sources (two separate sodium lamps, two lasers) are incoherent — phases fluctuate randomly with a correlation time of about $10^{-8}$ s for typical sources. Only coherent sources produce a stationary interference pattern visible to the eye.

Subtopic 5: Brewster's Law and Polarization

Polarization is direct proof of transverse wave nature. Unpolarized light has electric field vectors in all orientations perpendicular to propagation. A polaroid (Polarizer) transmits only one orientation, halving the intensity: $I = I_0/2$. Brewster's law: $\tan\theta_p = n$. At $\theta_p$, reflected ray is completely polarized perpendicular to the plane of incidence; reflected and refracted rays are perpendicular. NEET numerical: if $n = 1.732 = \sqrt{3}$, then $\tan\theta_p = \sqrt{3}$, so $\theta_p = 60°$.

Subtopic 6: Malus's Law

For plane-polarized light through an analyser at angle $\theta$: $I = I_0\cos^2\theta$. Key values: $\theta = 0°$ → $I = I_0$; $\theta = 45°$ → $I = I_0/2$; $\theta = 60°$ → $I = I_0/4$; $\theta = 90°$ → $I = 0$. Three-polaroid problem: after a polaroid ($I_0/2$), second at $45°$ ($I_0/4$), third at $90°$ to first, i.e., $45°$ to second ($I_0/8$). Without the middle polaroid, crossed polaroids give $I = 0$.