Each bead in the glass case acts as both a prism and a mirror, reflecting light and separating it into a spectrum of rainbow colors. When you shine the light on the case, white light enters each bead, reflects off the bead and leaves again.
White light is made up of many different colors of light.
A glass bead bends each of these colors at a different angle, dispersing the white light into its component colors. As a result every bead produces its own tiny spectrum or rainbow. However your eyes intercept only one of the colors produced by a single bead. The rest of the colors from the bead are bent so far that they never enter your eyes and you never see them. We'll be adding interesting info and links here.