Trap 1 — Three Polaroids (most common NEET trap):

• Wrong: "Two crossed polaroids block all light, so adding a third cannot help" → answer 0
• Correct: The middle polaroid rotates the polarization direction → some light passes P3. Answer: $I_0/8$.

Trap 2 — Fringe width in liquid:

• Wrong: "Immersing in liquid with $n = 1.5$ increases the speed of light → larger $\lambda$ → larger $\beta$"
• Correct: Speed of light decreases in denser medium → $\lambda' = \lambda/n$ → $\beta' = \beta/n$. Fringe width decreases.

Trap 3 — Diffraction minima vs. YDSE maxima:

• Wrong: "$a\sin\theta = n\lambda$ gives maxima in single slit diffraction"
• Correct: $a\sin\theta = n\lambda$ gives minima in single slit; $d\sin\theta = n\lambda$ gives maxima in YDSE.

Trap 4 — Central maximum in single slit:

• Wrong: "Single slit central maximum width = $\lambda D/a$"
• Correct: Central maximum width $= 2\lambda D/a$ (twice the secondary maxima width).

Trap 5 — Brewster's angle and transmitted beam:

• Wrong: "Both reflected and refracted beams are completely polarized at Brewster's angle"
• Correct: Only the reflected beam is completely polarized. Refracted beam is only partially polarized.