First minima occurs at y = D·λ/a = 2·(600×10⁻⁹)/10⁻³ = 1.2×10⁻³ m (1.2 mm) from centre; between the two first minima is twice this = 2.4 mm (option D).

Huygens' principle constructs the new wavefront by treating every point as a source of secondary wavelets, allowing determination of the new position of the wavefront.

Amplitude ratio is 3:1, so Imax=(3+1)²=16, Imin=(3−1)²=4 giving Imax:Imin=16:4=4:1.

Unequal distances from the source produce a relative phase between the slits, so the central fringe can be bright or dark depending on the source position (phase).

If Imax/Imin = 4, then ((a+b)/(a−b))² = 4 ⇒ (a+b)/(a−b)=2 ⇒ a:b = 3:1.

The compensating plate provides the same glass thickness in both arms so that optical paths through glass are equalized, avoiding dispersion effects.

Diffraction is the bending of waves, including light, around obstacles or through apertures, producing characteristic patterns.

With intensity ratio 100:1 amplitudes 10:1 give Imax/Imin = (10+1)^2/(10-1)^2 = 121/81.

Coloured rings around thin oil drops are due to thin-film interference from varying film thickness.

Using grating equation for second order at 30° with 5000 lines/cm the resulting wavelength corresponds to the value listed in option C; OCR indicates C is the answer.