IB Physics-Theme C-Paper 2-Qc#4- Interference of waves-Double slit- Snell's law

A beam of coherent monochromatic light from a distant galaxy is used in an optics experiment on Earth.
The beam is incident normally on a double slit. The distance between the slits is 0.300 mm. A screen is at a distance D from the slits. The diffraction angle θ is labelled. A series of dark and bright fringes appears on the screen. Explain how a dark fringe is
formed.
b. The wavelength of the beam as observed on Earth is 633.0 nm. The separation between a dark and a bright fringe on the screen is 4.50 mm. Calculate D.
c. The air between the slits and the screen is replaced with water. The refractive index of water is 1.33.Calculate the wavelength of the light in water.
d. State two ways in which the intensity pattern on the screen changes.
-The graph of variation of intensity with diffraction angle for this experiment is shown.
Calculate the angular separation between the central peak and the missing peak in the double-slit interference intensity pattern. State your answer to an appropriate number of significant figures.
e. Deduce, in mm, the width of one slit.
f. The wavelength of the light in the beam when emitted by the galaxy was 621.4 nm.
Explain, without further calculation, what can be deduced about the relative motion of the
galaxy and the Earth.
Topic 4 and topic 9 oscillation and waves:

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How do we know exactly that the galaxy is moving away from us?