The infinity mirror effect is achieved using two mirrors: a regular mirror with LEDs in front of it, and a second one-way mirror that reflects the LED image back onto the fully reflective mirror. The result is a series of smaller and smaller reflections that recede into an infinite distance.
They are amazing to behold, and not too challenging to put together. Here are some of my favorite infinity mirror projects, from the basic infinity mirror to more unusual applications: clocks, tables, pendants, and props.