Typical diffraction gratings commonly have on the order of hundreds to a few thousand lines per centimeter, so 200–2000 is appropriate.

Fringe spacing is proportional to the slit-to-screen distance, so doubling that distance doubles the fringe spacing.

Thomas Young's double-slit experiment (1801) provided clear verification of light interference, confirming wave behaviour.

Young's double-slit experiment demonstrates interference, which is characteristic of wave behaviour of light.

Intensity from each slit ∝ width, so amplitudes ∝ √I give amplitude ratio 1:3; I_min = (3-1)² =4, I_max=(3+1)²=16, ratio 4:16 = 1:4.

Rises and falls are beats; the number per second is the beat frequency.

Fringe width β = Dλ/d; with D→D/2 and d→2d, β' = (D/2)λ/(2d) = β/4, so it becomes one-fourth.

Diffraction angle is larger for longer wavelengths; thus an increased diffraction angle corresponds to an increased wavelength.

Fringe spacing β = λD/d; halving λ and halving d leaves β unchanged, so it remains 3.0 mm.

With intensities 4I and I, amplitudes 2 and 1 give Imax/Imin = (2+1)^2/(2-1)^2 = 9/1 = 9:1.