(a) The fact that light interferes means it’s a wave (this will be discussed more in the modern physics topic)
(b) Looking at the intensity pattern, P is the second point of zero intensity which means it’s the 2nd dark spot (m = 1.5).
Using Path Diff = mλ … m λ = dx / L … path diff = (2x10–3) (1.8x10–3) / 5 … path diff = 7.2x10–7m
(c) Path Diff = mλ … 7.2x10–7 = 1.5 λ … λ = 4.8x10–7 m
(d) (i) Based on n1 λ1 = n2 λ2 … nair λair = nwater λwater. The λwater is less in comparison to the air. So the λ has been decreased. In the equation m λ = d x / L, for decreased λ there will be decreased x, which means the location of spots is smaller, compressing the pattern.
(ii) Covering a slit makes this a single slit pattern. In the single slit, m = 1 becomes the first dark region (the end of the central bright spot) instead of m = 0.5 being the end of the central bright spot. All of the other integer m’s also become dark spot locations. The effect of this is to make the central max wider and widen the pattern. Additionally, in single slit diffraction, the intensities generally lose intensity more rapidly when moving away from the center.
(iii) Increasing d. Based on mλ = d x / L, this would make x less so would compress the pattern.