Phantom lightbulb: Image in a plane mirror.

Phantom lightbulb: Image in a plane mirror. In this demo, the light bulb at position A is switched on. It is clearly visible, glowing for the audience. The instructor then unscrews and removes the bulb. But the bulb is still there, glowing! (In the old days, the instructor would hit the bulb with a hammer, for more dramatic effect.) This is an illusion. The real light source is hidden at B behind a black metal cover. The dummy bulb at A is never turned on at all. Only the hidden bulb B is lighted when the switch is pressed. The bulb at A is the same distance from the pane of glass as the bulb at B, and on the same perpendicular to the glass as B. Thus the rays transmitted to the audience from bulb A coincide with the rays reflected from the bulb at B.