It sure sounds like he used the old trick of making an image and then
attempted to convince you that the image was the real thing. We found a
web site which has a picture of what we think you're talking about:
, but who knows how long web sites will stay before breaking their links.
Anyhow, the picture appears to be one with two candles, with a
sheet of clear glass between them. One candle is lit, and the other one
isn't. If you stand on the same side of the glass as the lit candle,
and stand in the right place, then the reflection of the light from the
burning candle's flame will line up with the light coming from the
other candle's unlit wick. The light from the flame reflects off of the
glass, while the light coming from the rest of the other candle comes
through the glass. Since glass both reflects light and lets light pass
through, you see light from both candles. Your teacher can then
capitalize on the illusion and put his hand over the candle that's not
lit and you will see the light from his finger and the light from the
flame approach you from the same direction, even though they didn't
come from the same place.
The reason glass reflects light is because of the mismatch between the refractive indexes in the air and the glass.
Mike and Tom
(republished on 07/29/06)