Novel optical properties of a submerged light bulb

When an unlit frosted light bulb is immersed in water, the inside frosting appears to shrink, and the outside surface of the bulb appears to become reflective. We explore these interesting effects using geometric optics at the level of introductory physics. It is found that rays emitted from the particles of powder that coat the inside surface of a frosted bulb emerge in conical beams when the bulb is submerged, which results in a reduction of the apparent size of the bulb's internal surface. The observed reflectivity is explained by the total internal reflection that occurs at the internal glass-gas interface. Light rays that refract through a clear bulb are analyzed to understand why a small spherical internal surface appears to exist in this case. The effects of bulb thickness, viewing height, and frustrated total internal reflection are also considered. © 2008 American Association of Physics Teachers.