Question Identification (First 30 Seconds)

Numerical — YDSE: Identify $\lambda$, $D$, $d$. If any one is changed (doubled, halved, put in medium), apply $\beta = \lambda D/d$ directly. Avoid long derivations; the answer is always a simple ratio.

Numerical — Polarization: Count the number of polaroids and angles. Apply $I_0/2$ at the first polaroid (unpolarized input), then $I_n = I_{n-1}\cos^2\theta$ at each subsequent one. Never apply $\cos^2$ to unpolarized light.

Numerical — Brewster: $\tan\theta_p = n$. If $n = \sqrt{3}$, $\theta_p = 60°$. If $n = 1$, $\theta_p = 45°$. Always verify $\theta_p + \theta_r = 90°$ as a sanity check.

Conceptual MCQ: Keywords to watch — "coherent sources" (same frequency + constant phase difference); "sustained interference" (requires coherence); "transverse wave" (proved by polarization); "central fringe in white light" (white, all wavelengths overlap at zero path difference).

Time Allocation

High-Yield Quick Saves (1-mark each)

• "Fringe width when $d$ is doubled?" → halved ($\beta \propto 1/d$)
• "YDSE immersed in water ($n = 4/3$)?" → $\beta$ becomes $3\beta/4$
• "What proves light is transverse?" → polarization
• "Light through crossed polaroids?" → zero intensity
• "Central maximum in single slit vs YDSE?" → single slit central is twice as wide