If you have Pozar 2nd ed, it's problem 1.7.
Details of the problem: plane wave is normally incident on a dielectric slab of permittivity er and thickness d=lambda0/(4*sqrt(er)). lambda0 is the free space wavelength of the incident wave. if free space exists on both sides of the slab, find the reflection coefficient of the wave reflected from the front of the slab.
Don't know how to insert pictures but it looks like this: free spacee (e0)/medium(0=>d,e0er)/free space(e0)...along the z axis.