What is the meaning of this "let there be light" joke?

As most people know, "let there be light" is a famous biblical quote, from Genesis. Now, on to the teacher's shirt.

Those equations on his back are Maxwell's equations. "Let there be light" is a joke, because Maxwell's equations describe electromagnetic fields, and light is a form of electromagnetic radiation, so the equations can be used to describe light.

So, as a physicist, one could (jokingly) say that God's "let there be light" refers to him 'inventing' Maxwell's equations.