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"It's a reciprocal lattice, Batman."
Joker cast a lazy glance over the textbook his nemesis had slammed onto the table. The aforementioned slammer leaned in and scowled through his cowl.
"It doesn’t make sense, Joker. How does it fit into the real lattice?"
Joker smacked his lips. It was going to be a long one.
"Well, most of the time in a crystal you only care about getting from one place to the next—but a wave in a crystal—well, they'll be doing the same thing over and over again all over the place!"
"What kind of wave, Joker?"
Joker smirked jokerously.
"Well, it depends, Batman—we're really getting ahead of ourselves here—"
A pair of armoured gauntlets grabbed Joker's lapels and dragged him bodily over the interrogation room table.
"What does the lattice mean, Joker?"
"Crystals are awfully big things, aren’t they, Batman? So if an atom in a crystal is reflecting or emitting a wave, chances are they have quite a few friends doing the same thing, hm? And if everyone's emitting the same wave, what do you get?"
Joker cackled wickedly.
"Nothing! All the peaks and troughs hit each other at random! Nobody can agree what phase the wave should be at any point in the lattice! Unless—"
He licked his lips.
"Unless the wavevector is a multiple of the spacing, Batman... then suddenly everyone sees it the same way... all the phases are coherent. The wavevector can be one spacing, two spacings, three—"
A blinding crack of semi-consciousness later, Joker realised he’d been slammed into the table again, and looked up at the Batman reproachfully.
"...on and on forever, in each direction the crystal's periodic in. Like some kind of Lattice. But not just any lattice, Batman, a Reci-"
It was at this point that the Joker's memories of the evening, alongside his respectable two-week no-concussion streak, concluded.