Parallel laser beam (=550 nm) falls perpendicularly on the opaque screen that has a circular opening with diameter D=2 mm. A dark spot is observed at a distance b1=18 cm on the central axis behind the screen. What is the minimal distance b that one has to move away from this spot and further away from the screen in order to observe a dark spot again?

So what is the question

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.