It really wasn’t that long ago that computers came equipped with optical disc drives, and they were effective means of data storage, and the density you could store on a DVD instead of a CD was pretty exciting. Now? They’re not only borderline-useless, but the features that we used to reference as a cultural touchstone are no longer obvious to our students.
It’s not that they don’t know what these are. It’s that they haven’t handled a million of them to know their dimensions, to understand the diffraction rainbow they make. The physicists around here remember using the inescapable AOL discs as cheap, readily available diffraction gratings back in grad school. The astronomers use their proportions to illustrate the shape of the Milky Way Galaxy. Students now need to physically hold one of these to get the idea, because they don’t have a mental image ready to go.
Our Galaxy, if you were wondering, is roughly proportioned as a CD, only instead of being a millimeter and a half thick, is more like 1,000 light years. Very roughly, anyway.