What is it all about?
The applet demonstrates the reflection and transmission for a layered structure with as a building block (1D 'unit cell') a layer with refractive index n1 and thichkness h1 and a second layer with refractive index n2 and thickness h2. This unit cell is repeated many times to gradually buildup a dielectric mirror with very high reflectivity and no absorption. The position of the reflection maximum depends on the choice for the material parameters.

Reflection from a layered medium has many features in common with Photonic Band Gap (PBG) materials. The high reflectivity region is similar to  the 'stop gap' of a PBG in certain directions. The main difference is that a true Photonic Band Gap material is a three dimensional periodic structure and has perfect reflection properties in many directions. Ultimately when the stop gaps overlap in all directions for some wavlengths a genuine photonic band gap exists.

, 7 may 1999.