Central angular width ≈ 2λ/a = 2×6328Å / 0.2 mm ≈ 0.00633 rad ≈ 0.363°, matching option A.

Interference requires coherence, i.e., a constant phase relationship between sources; proximity or width are not essential alone.

Coherent sources maintain a constant phase relationship, which is necessary for stable interference patterns.

The standard definition of refraction is the bending of light at the interface between two media with different optical densities.

Fringe width β = λD/d is independent of the fringe order; it depends on wavelength, screen distance and slit separation but not on which order is considered.

Fringe position y ∝ λ, so the smaller-wavelength blue light gives a smaller fringe displacement than green light, hence X(blue) < X(green).

Crest over crest and trough over trough reinforces amplitudes, which is constructive interference.

With intensity ratio 400:1, amplitude ratio is 20:1; Imax/Imin = (20+1)^2/(20-1)^2 = 21^2/19^2, so approximately 21/19, option C.

Reducing intensity of one slit decreases fringe contrast: maxima become less bright and minima become less dark, but fringes persist.

Imax/Imin = ((A1+A2)/(A1−A2))^2 = 4 gives (A1/A2) = 3, so amplitude ratio is 3:1.