Q & A: virtual images

My teacher conducted a demonstration that has me baffled. He had a lighted candle that was in front of a rectangular glass box. There appeared to be a candle in the glass box. Then he said that he had invented a burn proof substance. He slathered this substace over his hand, then placed his uncovered hand over the candle that was standing in front of the glass box. He showed that this was painful, then placed the hand that was covered with the "burn proof" substance over the candle in the glass box. He kept it there for a while claimimg that it was not at the least painful. How did my teacher do this? What scientific principles were involved? Was the candle in the glass box just a reflection?
- Kashan (age 14)
Central High School, Philadelphia

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

(published on 10/22/2007)