Okay, here's the correct answer. My initial impression was incorrect: [quote]If the inside of a cube were completely covered with perfect mirrors then light would bounce around inside the cube forever; however, with imperfect mirrors that reflect less than exactly 100% of all incident light, many successive reflections will eventually dissipate all the light. Because light moves very quickly, a light pulse will undergo a great many reflections in a short time (as long as the cube is not extraordinarily large), causing rapid dissipation; even inside a highly reflective cube, darkness will return almost immediately after a flash of light. For example, the best silver mirrors reflect up to 99% of the visible light that hits them. After ten successive reflections between such mirrors, 90% of the initial light-flash continues to bounce around the cube; after a hundred reflections 37% remains, but after a thousand reflections, less than 0.005% remains. Since light travels approximately 1000 feet per microsecond (one millionth of a second), a flash of light in a one-foot cube will suffer 1000 reflections in a mere microsecond; if the cube's walls reflect less than 99%, the light will dissipate in much less than a microsecond. The enormous speed of light thus keeps typical mirrors from trapping light for any reasonable length of time. If we could make mirrored cubes so reflective that light could be stored for hours, I'm sure you could dream up lots of wonderful applications (solar powered flashlights?) Unfortunately, mirrors don't reflect visible light well enough to store light for more than a microsecond; however, there are other kinds of electromagnetic waves that are more "reflectable" than visible light. (Visible light constitutes a small part of the spectrum of electromagnetic waves; x-rays, ultraviolet light, and radio waves are also kinds of electromagnetic waves). In the last several decades, we have found that some extraordinary materials can reflect 99.9999999% of incoming radio waves (although they still reflect less than 90% of visible light) and it is not inconceivable that someone might someday (probably not in the near future) make similarly excellent mirrors for visible light, at which time I'd expect your "mirrored cube" to become as familiar and important as the electric light bulb.[/quote]