What is the meaning of this "let there be light" joke? Someone across the restaurant is wearing this shirt, and I certainly don't get it.

Update
Related: What does this quote about the four dimensional divergence of an antisymmetric tensor mean?
 A: 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.
A: The Divergence of the Magnetic field (B) in vacuum is $0$. $$\nabla\cdot B=0$$
The Divergence of the Electrical field (E) in vacuum is $0$.$$\nabla\cdot E=0$$
The Curl of the Electric field in  Vacuum is equal to minus the rate of change of the magnetic field with time.$$\nabla\times E=-\frac{dB}{dt}$$
The Curl of the Magnetic field in Vacuum is equal to the Permeability of free space times the Permissivity of free space times the rate of change of the electrical field with time. $$\nabla\times B=\mu_0\nu_0\frac{dE}{dt}$$
That established:
The curl of the curl of a Vector field is equal to the Gradient of the Divergence of the Field minus the laplacian operator of that field $$\nabla(\nabla\cdot A)-\nabla^2A$$
so we have $$\nabla \times{\nabla\times E}=0-\nabla^2(E)$$
$$\nabla\times -\frac{dB}{dt}=-\frac{d(\nabla\times B)}{dt}$$
$$=- \mu_0\nu_0\frac{d^2(E)}{dt^2}$$
so we have $$\nabla^2{E}= \mu_0\nu_0 \frac{d^2(E)}{dt^2}$$
Which is the form of the wave equation.
Specifically, this tells us that it defines a wave with a speed of  over the square root of the permeability times the permissivity of free space.
Which is the speed of light:
This equation is thus the equation of the electrical component for the speed of light:
You can do exactly the same math for the magnetic field.
So we have the equations for light, predicting the speed of light, that were, incidentally, formulated before the speed of light was measured, and predicted it.
thus:
Let there be light.
