What do these variables represent in an equation for thin film reflectivity?

Homework Statement

I'm just trying to figure out what all the variables are in an equation I'm using for thin film reflectivity. Any advice beyond that would be appreciated though...

Homework Equations

rthin slab=(-i * $$\lambda$$ * $$\rho$$ * r0 * $$\Delta$$)/(sin($$\alpha$$))

note: alpha here isn't an exponent, it's sin(alpha), I just couldn't fix the formatting for some reason.

The Attempt at a Solution

rthin slab is the reflectivity. i is the imaginary number i. lambda is the wavelength. rho is the density. r0 is the Thomson scattering length. alpha is the angle the light makes with the surface of the thin film as it hits it. I'm not sure what delta is, though...does that have something to do with the size of the thin film? Because apparently that's supposed to be a part of this equation...I really appreciate any and all help!