When a laser beam reflects from a back surface glass mirror and falls on a screen, a pattern of discrete bright spots is created by partial reflection and refraction of the light at the air-glass interface and reflection at the mirror surface (Fig. 1). This paper explains how this phenomenon can be used to determine the refractive index and the thickness of the glass with a simple measurement. It is possible to utilize this experiment for geometrical optics labs and moreover it would be a nice practice for Physics Olympians.